I have constructed a Mixer for a VHF receiver (150MHz).
The IF frequency is 45 MHz and LO is 105 MHz.

Having not made a mixer before, I copied a typology directly from one of
my fundamental electronics books.

The layout is...
RF signal into Gate 2 of BF998.
Source has 100R Par with 1n to ground.
G1 has 47K to ground and 100p to LO signal (I am using a sig gen for
testing).

The drain has a Tuned circuit consisting of 100nH and 125pF.
A 1k resistor is across the coil to provide approx. 800 Ohms needed
to interface to a 4-pole crystal filter.

For testing I used a tap-C down to 50 Ohms from the 800R tank in the
drain of the mixer BF998.

I observe the frequencies coming out of the mixer when I inject the 150
MHz RF signal and find I can tune the 45MHz for a peak no problem.
There do not appear to be any instabilities.
The gain of the preceding stages (Input single tuned filter, MOSFET RF
amp and Double Tuned Circuit) is around 24dB.

When I add the mixer I find the total system gain is the same (24dB),
The mixer seems to be mixing but adding no gain or loss. I also wonder
where the mixer MOSFET is getting bias (from the LO ?) because the mixer
stage doesn't appear to be drawing any current. I found the max. gain at
the mixer output was when the LO level was anywhere from -5 to 0dBm.

Would someone please mind explaining how the bias of the mixer should be
applied and how much gain/loss I should expect. ( i hope I am asking the
right questions here as have no previous experience with a discrete mixer).

Thank in advance.

Regards

David

Andy comments:

I haven't used that particular dual gate Mosfet, but I will tell you
how I have used 3n140, 3n141, and others:

G2 was used as the LO and was bias at around 3 volts DC with
resistors. The LO was cap coupled in at a leveL of about 2.5 Vrms.
It is useD as a switching signal to drive the MOSFET into and out
of conduction. It takes very little power from the LO, but the
2.5 Vrms swing was needed...

G1 was used as the RF port. G1 and the source were biased
pretty much as you described.

The drain was similar to your design. While using a 1K resistor
gives
a great source impedance to the xtal filter, it also absorbs most of
the signal.... If I remember, the output Z is 9 or 10 k for the devices
I used. I would use maybe a 7.5 K resistor and then impedance
match in the tuned circuit to get the source right. This usually took
some experimentation. Since the drain output was tuned to a different
frequency than the LO or the RFin, I never had a stability problem.
Power gains of 15 to 20 db were very common... ( Note , I said
POWER gain, and not voltage gain , which could be anything... )



Maybe my experience is outdated with the newer Mosfets, but I
have never seen, or used a circuit that put the LO on G!.... It was
always on G2.. I used dual gate mosfets to build receiver front
ends and mixer from about 1967 to about 1990. Then I started
using doubly balanced stuff like SBLs and MDs ..... I followed them
with a nice amplifer and good termination.....It costs a little more,
but the designs were always more stable and gave better
performance.....

I hope this helps. Perhaps others here have used your particular
device and can give better guidance...

Andy W4OAH

On 2 Jul 2006 10:27:37 -0700, "AndyS" wrote:


Andy comments:

I haven't used that particular dual gate Mosfet, but I will tell you
how I have used 3n140, 3n141, and others:

G2 was used as the LO and was bias at around 3 volts DC with
resistors. The LO was cap coupled in at a leveL of about 2.5 Vrms.
It is useD as a switching signal to drive the MOSFET into and out
of conduction. It takes very little power from the LO, but the
2.5 Vrms swing was needed...

G1 was used as the RF port. G1 and the source were biased
pretty much as you described.

The drain was similar to your design. While using a 1K resistor
gives
a great source impedance to the xtal filter, it also absorbs most of
the signal.... If I remember, the output Z is 9 or 10 k for the devices
I used. I would use maybe a 7.5 K resistor and then impedance
match in the tuned circuit to get the source right. This usually took
some experimentation. Since the drain output was tuned to a different
frequency than the LO or the RFin, I never had a stability problem.
Power gains of 15 to 20 db were very common... ( Note , I said
POWER gain, and not voltage gain , which could be anything... )



Maybe my experience is outdated with the newer Mosfets, but I
have never seen, or used a circuit that put the LO on G!.... It was
always on G2.. I used dual gate mosfets to build receiver front
ends and mixer from about 1967 to about 1990. Then I started
using doubly balanced stuff like SBLs and MDs ..... I followed them
with a nice amplifer and good termination.....It costs a little more,
but the designs were always more stable and gave better
performance.....

I hope this helps. Perhaps others here have used your particular
device and can give better guidance...

Andy W4OAH

Your experience is like mine and holds well for current generation
DGfets or even a pair of casoded Jfets.

Allison

I noted in We Hayward's book "Experimental Methods in RF Design" that he
suggests the gain of a Dual Gate Mosfet in a mixer circuit is about 1/4
of the gain of the same device in an RF amp circuit. If this is true
then the 6dB gain I now see in the mixer stage would be about right.

I cranked up the LO level and now have +/- 4V on G2. I tried biasing the
Gate up to 3 but it made no difference than when just 47K to ground
bias was used on G2.

The main issue I am having now is matching to the 4-pole filter.
I have tried several approaches and the performance is disgusting.

The current mixer to filter circuit is....

This "should" have matched down to 800R

100nH (Q=100) inductor to Vcc from Drain
6-30p trimmer to ground from drain
Split capacitor tap from drain to ground (220pF in series with 200pF)

1st filter, 4p7 to ground between 1st and second filter.

Output match to 50R from 800R for testing...
100nH to ground from filter output
82pF to ground
6-30p trimmer to ground
15pF in series to 50 Ohm load.

The loss through the filter is around 10dB instead of 3dB, the ripple is
around 6-8db instead of 1dB. The filter response shows double peaks with
dip between, either side of the peaks falls off extremely quickly at
around 2 kHz off (should be +/- 15kHz bandwidth).

I would appreciate any help I can get to determine what is happening and
to correctly match into this filter that requires 800R//3pF terminations
at 45 MHz.


AndyS wrote:
Andy comments:

I haven't used that particular dual gate Mosfet, but I will tell you
how I have used 3n140, 3n141, and others:

G2 was used as the LO and was bias at around 3 volts DC with
resistors. The LO was cap coupled in at a leveL of about 2.5 Vrms.
It is useD as a switching signal to drive the MOSFET into and out
of conduction. It takes very little power from the LO, but the
2.5 Vrms swing was needed...

G1 was used as the RF port. G1 and the source were biased
pretty much as you described.

The drain was similar to your design. While using a 1K resistor
gives
a great source impedance to the xtal filter, it also absorbs most of
the signal.... If I remember, the output Z is 9 or 10 k for the devices
I used. I would use maybe a 7.5 K resistor and then impedance
match in the tuned circuit to get the source right. This usually took
some experimentation. Since the drain output was tuned to a different
frequency than the LO or the RFin, I never had a stability problem.
Power gains of 15 to 20 db were very common... ( Note , I said
POWER gain, and not voltage gain , which could be anything... )



Maybe my experience is outdated with the newer Mosfets, but I
have never seen, or used a circuit that put the LO on G!.... It was
always on G2.. I used dual gate mosfets to build receiver front
ends and mixer from about 1967 to about 1990. Then I started
using doubly balanced stuff like SBLs and MDs ..... I followed them
with a nice amplifer and good termination.....It costs a little more,
but the designs were always more stable and gave better
performance.....

I hope this helps. Perhaps others here have used your particular
device and can give better guidance...

Andy W4OAH

On Sun, 02 Jul 2006 22:45:57 GMT, David
wrote:

Since everyone is top posting.

Having G2 driven hard is good for large signal handling as mixing
will occure with lesser levels. So that's why that needs to be done.
If driven hard enough bias has no effect.

I vaugely remember you saying your using two filters back to back...

If so you may have the interfilter coupling way off. Also the loss for
two 3db loss filters would be a bit higher than 6db if everything is
right.

Allison




I noted in We Hayward's book "Experimental Methods in RF Design" that he
suggests the gain of a Dual Gate Mosfet in a mixer circuit is about 1/4
of the gain of the same device in an RF amp circuit. If this is true
then the 6dB gain I now see in the mixer stage would be about right.

I cranked up the LO level and now have +/- 4V on G2. I tried biasing the
Gate up to 3 but it made no difference than when just 47K to ground
bias was used on G2.

The main issue I am having now is matching to the 4-pole filter.
I have tried several approaches and the performance is disgusting.

The current mixer to filter circuit is....

This "should" have matched down to 800R

100nH (Q=100) inductor to Vcc from Drain
6-30p trimmer to ground from drain
Split capacitor tap from drain to ground (220pF in series with 200pF)

1st filter, 4p7 to ground between 1st and second filter.

Output match to 50R from 800R for testing...
100nH to ground from filter output
82pF to ground
6-30p trimmer to ground
15pF in series to 50 Ohm load.

The loss through the filter is around 10dB instead of 3dB, the ripple is
around 6-8db instead of 1dB. The filter response shows double peaks with
dip between, either side of the peaks falls off extremely quickly at
around 2 kHz off (should be +/- 15kHz bandwidth).

I would appreciate any help I can get to determine what is happening and
to correctly match into this filter that requires 800R//3pF terminations
at 45 MHz.


AndyS wrote:
Andy comments:

I haven't used that particular dual gate Mosfet, but I will tell you
how I have used 3n140, 3n141, and others:

G2 was used as the LO and was bias at around 3 volts DC with
resistors. The LO was cap coupled in at a leveL of about 2.5 Vrms.
It is useD as a switching signal to drive the MOSFET into and out
of conduction. It takes very little power from the LO, but the
2.5 Vrms swing was needed...

G1 was used as the RF port. G1 and the source were biased
pretty much as you described.

The drain was similar to your design. While using a 1K resistor
gives
a great source impedance to the xtal filter, it also absorbs most of
the signal.... If I remember, the output Z is 9 or 10 k for the devices
I used. I would use maybe a 7.5 K resistor and then impedance
match in the tuned circuit to get the source right. This usually took
some experimentation. Since the drain output was tuned to a different
frequency than the LO or the RFin, I never had a stability problem.
Power gains of 15 to 20 db were very common... ( Note , I said
POWER gain, and not voltage gain , which could be anything... )



Maybe my experience is outdated with the newer Mosfets, but I
have never seen, or used a circuit that put the LO on G!.... It was
always on G2.. I used dual gate mosfets to build receiver front
ends and mixer from about 1967 to about 1990. Then I started
using doubly balanced stuff like SBLs and MDs ..... I followed them
with a nice amplifer and good termination.....It costs a little more,
but the designs were always more stable and gave better
performance.....

I hope this helps. Perhaps others here have used your particular
device and can give better guidance...

Andy W4OAH

David wrote:
I noted in We Hayward's book "Experimental Methods in RF Design" that he
suggests the gain of a Dual Gate Mosfet in a mixer circuit is about 1/4
of the gain of the same device in an RF amp circuit. If this is true
then the 6dB gain I now see in the mixer stage would be about right.

I cranked up the LO level and now have +/- 4V on G2. I tried biasing the
Gate up to 3 but it made no difference than when just 47K to ground
bias was used on G2.

The main issue I am having now is matching to the 4-pole filter.
I have tried several approaches and the performance is disgusting.

The current mixer to filter circuit is....

This "should" have matched down to 800R

100nH (Q=100) inductor to Vcc from Drain
6-30p trimmer to ground from drain
Split capacitor tap from drain to ground (220pF in series with 200pF)

1st filter, 4p7 to ground between 1st and second filter.

Output match to 50R from 800R for testing...
100nH to ground from filter output
82pF to ground
6-30p trimmer to ground
15pF in series to 50 Ohm load.

The loss through the filter is around 10dB instead of 3dB, the ripple is
around 6-8db instead of 1dB. The filter response shows double peaks with
dip between, either side of the peaks falls off extremely quickly at
around 2 kHz off (should be +/- 15kHz bandwidth).

I would appreciate any help I can get to determine what is happening and
to correctly match into this filter that requires 800R//3pF terminations
at 45 MHz.


Andy writes:

Ok. Well, using +/- 4 volts ( i am guessing rms) will certainly
drive
the mosfet from full on to cutoff so DC biasing wouldn't be required.
Less LO could be used with a DC bias, but if you have the LO power
available, there is no reason to change.

I am assuming that you have gotten rid of the 800 drain load.....
The split cap approach is fine and should give you about a 4/1
step down, so if the drain output imp is 3200 ohms or thereabout, the
match should be close... ( I am doing this in my head, so forgive me if

I am off by a thousandfold :))) )

Now, the filter loss is something else. You did not tell me how you

measured it, and I am assuming that you just measured the voltages
and used that. This is a common mistake as there can be substantial
impedance change. The only accurate way to characterize filter loss
is with a special test jig which can measure the power into the load
without the filter and then the power into the load WITH the filter,
without
changing any of the tuning.... A purely resistive jig with a highZ
probe
can be fairly accurate, measuring voltage loss..... But you have to
allow
for any impedance changes in source and load.......

The inter match between the two filters should have both and L and
a C
making a parallel tank to make sure both the filter reactance, and the
stray reactances are tuned out. Usually filters are slightly
capacitive
, a couple pf, on their parallel terminal impedance......a tuned
circuit
will tune all this out..... giving you 800 to 800... You tune the tank
for minimum ripple in the passband... not for max gain at one of
the ripples.... tho it will be close...

The output transformation from 50 ohms up to 800 ohms represents
a 4 to 1 voltage transformation, or 12 db voltage loss..... In other
words,
if you used an 800 ohm resistive load (with a parallel tank to get rid
of the reactance and strays) you would, with a voltage probe, measure
12 db higher than you would when you transformed the load down to
50 ohms.... If you already knew this, and accounted for it, I
apologize for
assuming you didn't,, , but it is a common mistake some people have
made....

I don't disagree with Wes's book or explanation at all, but I am
surprised
that 12 db was the best the mosfet would do.....but, as I said, I have
never
used that particular one...

Remember, you have to match the RF generator UP to the input
Z of G1 in order to calculate the conversion gain. If you have just
connected G1 to a 50 ohm RF source, you are losing a lot of voltage
since the input Z of G! is probably a couple K..... That is quite a
voltage gain.... and that is before the conversion process even
starts....

It looks to me like you are on the right track. Again , I apologize
if it seemed
I was "talking down" to you, but I am just doing an all-purpose memory
dump
of all I remember about when I did this..... And I know for sure that

I got a hell of a lot more conversion gain out of a 3n141..... after
matching
both the input to G1 AND the output to a matched load....

Goodluck,,

Andy W4OAH

Andy,

The input to the mixer on G1 is the second stage of a DTC. The DTC was
designed to provide the appropriate RP for a 6MHz bandwidth. I then
assumed that G1 will be extremely high impedance compared to the DTC
and therefore would not load it down.

The gain was measured by first measuring the power output of the mixer
and then measuring the power output at the end of the filter.

The manufacturer data for the filter specifies maximum loss of 3dB (they
supply 2 matches filters that connect to form a 4-pole).

The output of the filter was stepped down to 50 Ohms with a C-Tap
impedance transform on the tuned circuit.

The input was matched from 2800 Ohms (Rp of the coil in the mixer tank)
to 800 R using a C-Tap transform also.

I have since constructed a stand alone filter test jig for the filter
only with match on input from 50 Ohms up to 800 and 800 back to 50.

I calibrate the output level with bypass across filter first for
reference level on the spectrum analyzer and then include the filter
in the path.
With this method I measure 4.8 dB loss and the double peaking action has
gone. The 1dB bandwidth was measured at +/- 9kHz.
When I sweep across the required bandwidth (+/-15kHz) I get 2.8dB loss
at the low end and 7.5dB at the high end (relative to the centre peak).
The tuning caps are able to peak the circuit without being turned to max
or min c, so I expect the tuned circuits are resonating ok.

Regards

David



AndyS wrote:
David wrote:
I noted in We Hayward's book "Experimental Methods in RF Design" that he
suggests the gain of a Dual Gate Mosfet in a mixer circuit is about 1/4
of the gain of the same device in an RF amp circuit. If this is true
then the 6dB gain I now see in the mixer stage would be about right.

I cranked up the LO level and now have +/- 4V on G2. I tried biasing the
Gate up to 3 but it made no difference than when just 47K to ground
bias was used on G2.

The main issue I am having now is matching to the 4-pole filter.
I have tried several approaches and the performance is disgusting.

The current mixer to filter circuit is....

This "should" have matched down to 800R

100nH (Q=100) inductor to Vcc from Drain
6-30p trimmer to ground from drain
Split capacitor tap from drain to ground (220pF in series with 200pF)

1st filter, 4p7 to ground between 1st and second filter.

Output match to 50R from 800R for testing...
100nH to ground from filter output
82pF to ground
6-30p trimmer to ground
15pF in series to 50 Ohm load.

The loss through the filter is around 10dB instead of 3dB, the ripple is
around 6-8db instead of 1dB. The filter response shows double peaks with
dip between, either side of the peaks falls off extremely quickly at
around 2 kHz off (should be +/- 15kHz bandwidth).

I would appreciate any help I can get to determine what is happening and
to correctly match into this filter that requires 800R//3pF terminations
at 45 MHz.


Andy writes:

Ok. Well, using +/- 4 volts ( i am guessing rms) will certainly
drive
the mosfet from full on to cutoff so DC biasing wouldn't be required.
Less LO could be used with a DC bias, but if you have the LO power
available, there is no reason to change.

I am assuming that you have gotten rid of the 800 drain load.....
The split cap approach is fine and should give you about a 4/1
step down, so if the drain output imp is 3200 ohms or thereabout, the
match should be close... ( I am doing this in my head, so forgive me if

I am off by a thousandfold :))) )

Now, the filter loss is something else. You did not tell me how you

measured it, and I am assuming that you just measured the voltages
and used that. This is a common mistake as there can be substantial
impedance change. The only accurate way to characterize filter loss
is with a special test jig which can measure the power into the load
without the filter and then the power into the load WITH the filter,
without
changing any of the tuning.... A purely resistive jig with a highZ
probe
can be fairly accurate, measuring voltage loss..... But you have to
allow
for any impedance changes in source and load.......

The inter match between the two filters should have both and L and
a C
making a parallel tank to make sure both the filter reactance, and the
stray reactances are tuned out. Usually filters are slightly
capacitive
, a couple pf, on their parallel terminal impedance......a tuned
circuit
will tune all this out..... giving you 800 to 800... You tune the tank
for minimum ripple in the passband... not for max gain at one of
the ripples.... tho it will be close...

The output transformation from 50 ohms up to 800 ohms represents
a 4 to 1 voltage transformation, or 12 db voltage loss..... In other
words,
if you used an 800 ohm resistive load (with a parallel tank to get rid
of the reactance and strays) you would, with a voltage probe, measure
12 db higher than you would when you transformed the load down to
50 ohms.... If you already knew this, and accounted for it, I
apologize for
assuming you didn't,, , but it is a common mistake some people have
made....

I don't disagree with Wes's book or explanation at all, but I am
surprised
that 12 db was the best the mosfet would do.....but, as I said, I have
never
used that particular one...

Remember, you have to match the RF generator UP to the input
Z of G1 in order to calculate the conversion gain. If you have just
connected G1 to a 50 ohm RF source, you are losing a lot of voltage
since the input Z of G! is probably a couple K..... That is quite a
voltage gain.... and that is before the conversion process even
starts....

It looks to me like you are on the right track. Again , I apologize
if it seemed
I was "talking down" to you, but I am just doing an all-purpose memory
dump
of all I remember about when I did this..... And I know for sure that

I got a hell of a lot more conversion gain out of a 3n141..... after
matching
both the input to G1 AND the output to a matched load....

Goodluck,,

Andy W4OAH

Andy,

I realise when I re-read your post that I have not included the loss of
the tuned circuit each side of the filter due to Rp of the inductors.

Taking this into account, the max. loss should be around 1.1dB per tuned
circuit + 3dB max. for filter = 5.2dB.

I re-measured the response using the digital power meter as this is more
accurate than the spectrum analyser relative measurements.

The insertion loss of filter AND tuned circuits was measured at 3.7dB

I then swept the filter and measured the -3dB bandwidth and found this
to be -18kHz, + 15.5kHz (very close to the expected +/- 15 kHz).
Next I measured stop band attenuation at +/- 60kHz and measured -47.3 dB
on low side and -50.7 dB on high side (manufacture data was for at least
40dB).

I then looked at ripple in the pass band. There was 1 ripple that
produces a dip at +9kHz of 1.6dB and a slight peak at +13 kHz of 0.86dB.

This is very close to the 1dB figure mentioned in the datasheet. There
was no ripple below Fo.

If I can now get the impedance transformation in my circuit working the
same as this then I'm on my way.

I'll leave the output match as-is and if there are issues then it
eliminates the output load and just leaves work to do at the mixer end.

This is the first discrete mixer and crystal filter I have constructed.
I'm not sure if I am overlooking anything here but to me those figures
now look good.

Does this all sound reasonable ?

Thanks heaps

Regards

David



AndyS wrote:
David wrote:
I noted in We Hayward's book "Experimental Methods in RF Design" that he
suggests the gain of a Dual Gate Mosfet in a mixer circuit is about 1/4
of the gain of the same device in an RF amp circuit. If this is true
then the 6dB gain I now see in the mixer stage would be about right.

I cranked up the LO level and now have +/- 4V on G2. I tried biasing the
Gate up to 3 but it made no difference than when just 47K to ground
bias was used on G2.

The main issue I am having now is matching to the 4-pole filter.
I have tried several approaches and the performance is disgusting.

The current mixer to filter circuit is....

This "should" have matched down to 800R

100nH (Q=100) inductor to Vcc from Drain
6-30p trimmer to ground from drain
Split capacitor tap from drain to ground (220pF in series with 200pF)

1st filter, 4p7 to ground between 1st and second filter.

Output match to 50R from 800R for testing...
100nH to ground from filter output
82pF to ground
6-30p trimmer to ground
15pF in series to 50 Ohm load.

The loss through the filter is around 10dB instead of 3dB, the ripple is
around 6-8db instead of 1dB. The filter response shows double peaks with
dip between, either side of the peaks falls off extremely quickly at
around 2 kHz off (should be +/- 15kHz bandwidth).

I would appreciate any help I can get to determine what is happening and
to correctly match into this filter that requires 800R//3pF terminations
at 45 MHz.


Andy writes:

Ok. Well, using +/- 4 volts ( i am guessing rms) will certainly
drive
the mosfet from full on to cutoff so DC biasing wouldn't be required.
Less LO could be used with a DC bias, but if you have the LO power
available, there is no reason to change.

I am assuming that you have gotten rid of the 800 drain load.....
The split cap approach is fine and should give you about a 4/1
step down, so if the drain output imp is 3200 ohms or thereabout, the
match should be close... ( I am doing this in my head, so forgive me if

I am off by a thousandfold :))) )

Now, the filter loss is something else. You did not tell me how you

measured it, and I am assuming that you just measured the voltages
and used that. This is a common mistake as there can be substantial
impedance change. The only accurate way to characterize filter loss
is with a special test jig which can measure the power into the load
without the filter and then the power into the load WITH the filter,
without
changing any of the tuning.... A purely resistive jig with a highZ
probe
can be fairly accurate, measuring voltage loss..... But you have to
allow
for any impedance changes in source and load.......

The inter match between the two filters should have both and L and
a C
making a parallel tank to make sure both the filter reactance, and the
stray reactances are tuned out. Usually filters are slightly
capacitive
, a couple pf, on their parallel terminal impedance......a tuned
circuit
will tune all this out..... giving you 800 to 800... You tune the tank
for minimum ripple in the passband... not for max gain at one of
the ripples.... tho it will be close...

The output transformation from 50 ohms up to 800 ohms represents
a 4 to 1 voltage transformation, or 12 db voltage loss..... In other
words,
if you used an 800 ohm resistive load (with a parallel tank to get rid
of the reactance and strays) you would, with a voltage probe, measure
12 db higher than you would when you transformed the load down to
50 ohms.... If you already knew this, and accounted for it, I
apologize for
assuming you didn't,, , but it is a common mistake some people have
made....

I don't disagree with Wes's book or explanation at all, but I am
surprised
that 12 db was the best the mosfet would do.....but, as I said, I have
never
used that particular one...

Remember, you have to match the RF generator UP to the input
Z of G1 in order to calculate the conversion gain. If you have just
connected G1 to a 50 ohm RF source, you are losing a lot of voltage
since the input Z of G! is probably a couple K..... That is quite a
voltage gain.... and that is before the conversion process even
starts....

It looks to me like you are on the right track. Again , I apologize
if it seemed
I was "talking down" to you, but I am just doing an all-purpose memory
dump
of all I remember about when I did this..... And I know for sure that

I got a hell of a lot more conversion gain out of a 3n141..... after
matching
both the input to G1 AND the output to a matched load....

Goodluck,,

Andy W4OAH

On 2 Jul 2006 16:31:27 -0700, "AndyS" wrote:


David wrote:
I noted in We Hayward's book "Experimental Methods in RF Design" that he
suggests the gain of a Dual Gate Mosfet in a mixer circuit is about 1/4
of the gain of the same device in an RF amp circuit. If this is true
then the 6dB gain I now see in the mixer stage would be about right.

I cranked up the LO level and now have +/- 4V on G2. I tried biasing the
Gate up to 3 but it made no difference than when just 47K to ground
bias was used on G2.

The main issue I am having now is matching to the 4-pole filter.
I have tried several approaches and the performance is disgusting.

The current mixer to filter circuit is....

This "should" have matched down to 800R

100nH (Q=100) inductor to Vcc from Drain
6-30p trimmer to ground from drain
Split capacitor tap from drain to ground (220pF in series with 200pF)

1st filter, 4p7 to ground between 1st and second filter.

Output match to 50R from 800R for testing...
100nH to ground from filter output
82pF to ground
6-30p trimmer to ground
15pF in series to 50 Ohm load.

The loss through the filter is around 10dB instead of 3dB, the ripple is
around 6-8db instead of 1dB. The filter response shows double peaks with
dip between, either side of the peaks falls off extremely quickly at
around 2 kHz off (should be +/- 15kHz bandwidth).

I would appreciate any help I can get to determine what is happening and
to correctly match into this filter that requires 800R//3pF terminations
at 45 MHz.


Andy writes:

Ok. Well, using +/- 4 volts ( i am guessing rms) will certainly
drive
the mosfet from full on to cutoff so DC biasing wouldn't be required.
Less LO could be used with a DC bias, but if you have the LO power
available, there is no reason to change.

You need to drive it hard for best overload performance.

DGfets were a favorite before DBMs and other high level mixers
or Gilbert cells.

I am assuming that you have gotten rid of the 800 drain load.....
The split cap approach is fine and should give you about a 4/1
step down, so if the drain output imp is 3200 ohms or thereabout, the
match should be close... ( I am doing this in my head, so forgive me if

I am off by a thousandfold :))) )

You likely right on. I used 5k as a round number years ago with
success with 3n203s and the BF998 devices have higher GM and
Idss so that would fit with your number.

If you load it low the gain drops but overload and noise performance
is not hurt badly.


Now, the filter loss is something else. You did not tell me how you

measured it, and I am assuming that you just measured the voltages
and used that. This is a common mistake as there can be substantial
impedance change. The only accurate way to characterize filter loss
is with a special test jig which can measure the power into the load
without the filter and then the power into the load WITH the filter,
without
changing any of the tuning.... A purely resistive jig with a highZ
probe
can be fairly accurate, measuring voltage loss..... But you have to
allow
for any impedance changes in source and load.......

The other nasty is most of those filters are more sensitive to
reactive loading than errors is resistive loading. So the scope
probe C (cheap probes can be 7-10pf) can be a real factor
in both meaurement error and circuit loading(capacitive termination).

The inter match between the two filters should have both and L and
a C
making a parallel tank to make sure both the filter reactance, and the
stray reactances are tuned out. Usually filters are slightly
capacitive
, a couple pf, on their parallel terminal impedance......a tuned
circuit
will tune all this out..... giving you 800 to 800... You tune the tank
for minimum ripple in the passband... not for max gain at one of
the ripples.... tho it will be close...

The output transformation from 50 ohms up to 800 ohms represents
a 4 to 1 voltage transformation, or 12 db voltage loss..... In other
words,
if you used an 800 ohm resistive load (with a parallel tank to get rid
of the reactance and strays) you would, with a voltage probe, measure
12 db higher than you would when you transformed the load down to
50 ohms.... If you already knew this, and accounted for it, I
apologize for
assuming you didn't,, , but it is a common mistake some people have
made....

I don't disagree with Wes's book or explanation at all, but I am
surprised
that 12 db was the best the mosfet would do.....but, as I said, I have
never used that particular one...

The fet is a square law mixer so gain and conversion figures are
not the same as if it were straight amplifer. The amplifier form is
two cascaded gain stages and the mixer form is an amplifer with a
series switch, least that model works for me.

Remember, you have to match the RF generator UP to the input
Z of G1 in order to calculate the conversion gain. If you have just
connected G1 to a 50 ohm RF source, you are losing a lot of voltage
since the input Z of G! is probably a couple K..... That is quite a
voltage gain.... and that is before the conversion process even
starts....

It looks to me like you are on the right track. Again , I apologize
if it seemed
I was "talking down" to you, but I am just doing an all-purpose memory
dump
of all I remember about when I did this..... And I know for sure that
I got a hell of a lot more conversion gain out of a 3n141..... after
matching
both the input to G1 AND the output to a matched load....


Big time! I found that the '141 and friends like to see around 1-2k
impedence level for best gain and at less than 600 ohms it was
pretty poor. Noise figure also improves.


Allison


Goodluck,,

Andy W4OAH

David wrote:

Andy,

The input to the mixer on G1 is the second stage of a DTC. The DTC was
designed to provide the appropriate RP for a 6MHz bandwidth. I then
assumed that G1 will be extremely high impedance compared to the DTC
and therefore would not load it down.

The gain was measured by first measuring the power output of the mixer
and then measuring the power output at the end of the filter.

The manufacturer data for the filter specifies maximum loss of 3dB (they
supply 2 matches filters that connect to form a 4-pole).

The output of the filter was stepped down to 50 Ohms with a C-Tap
impedance transform on the tuned circuit.

The input was matched from 2800 Ohms (Rp of the coil in the mixer tank)
to 800 R using a C-Tap transform also.

-- snip --

This could be your problem right here -- at 45MHz it's probably not
realistic to expect no loss in the mixer drain circuit. I'd find the Q
of the mixer tank without the filter, and use that to calculate the
effective parallel resistance, as well as the attenuation of the filter.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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