On Mar 13, 12:02 pm, "Anthony Fremont" wrote:

I believe this is direct conversion and that the crystal should exactly
match the desired station (3581kHz) I want to recieve, that's right isn't
it? Maybe this is close enough? I have to wait til tonight to see if I can
actually hear anything.

Depends on what sort of signal you are trying to receive.

To receive a single our double sideband signal (or, in a very touchy
way, an AM one), you want your direct conversion receiver's local
oscillator exactly on the transmitter frequency.

But to receive a CW signal, you want your local oscillator 700 Hz or
so above or below the transmit frequency. That will cause you to hear
a 700 Hz audio tone for the CW.

Unless I'm mistaken, 3.581 MHz is in the CW portion of the 80 meter
band.
If you can get to 1 KHz below the transmitter, you should be able to
copy CW as 1KHz audio. If you can only get to 1.5 KHz below, you'll
get 1.5 KHz audio... not ideal listening, but probably workable.

On 14 Mar, 07:42, "Anthony Fremont" wrote:
Anthony Fremont wrote:
Doodling with reactance formulas, it appears that 20uH (coincidence?)
would offset 100pF of capacitance fairly well by having a an opposing
reactance (well resistance at this point) of about 450 Ohms at

s/.well resistance at this point.//
It's just inductive reactance, I need more coffee. ;-)

3581kHz, the same as 100pF. I'll try putting the coil I made in
parallel and see what happens. Hopefully it won't short the
oscillator and kill my 15 year old NE602, I only have two spares.

Should I be afraid to do this? Does it need something to block DC current?

Anthony

Your trimmer cap will block any DC flow to ground. If you are really
paranoid then put a 0.1 mfd in series with the crystal.

If you do accidentally ground the crystal input terminal on that
SA602, it only pulls the base of a transistor to ground and thus turns
it off. That shouldn't hurt anything.

While you may not be able to purchase new "602's" now (they are long
ago declared obsolete) the Phillips SA-612 is the same unit and is
readily available from a number of outlets.

A datasheet for this device is located at:

http://www.nxp.com/pip/SA612AD_01.html

Arv - K7HKL
_._

colin wrote:
"Anthony Fremont" wrote in message
...
colin wrote:

Its only a pull of ~100ppm, this should be easily pullable for most
fundamental xtals.

I'd like to get close to 500ppm if possible.

do you need that much ?

I "need" an increase of about .0546%. Isn't that about 546ppm?

what freq you need ?

3581.5kHz to zero beat with the desired signal.

whats the colourburst crystal freq ?

3579.545kHz.


ofc if the tolerenace all add up against you you might find it hard.

I would also try reduce the 100pf caps on the sa602 too.
100pf is higher than most crystals I use would like,
50pf or 20pf or if youve got some spare trimmers ...
you can adjust the ratio too, say just reduce the one accross pin
6-7

I figured that they were voltage dividers to set the amount of
feedback, but I can certainly see how they could have an effect. Since
I'm close to where I need to be, I will try a couple of 33pF
caps to see what happens.

no they are involved in setting the frequency too,
in order for you circuit to work it needs to resonate,
with the 2 100pf the input where the crystal is looks like a
capacitor with some negative impedance,
the circuit with your crystal, inductor, and trimmer must be
inductive, it then forms a resonant ciruit with the capacitance of the
input.
usualy the crystal would just be operated so that it looks inductive.

Ok, I had to read that a few times to get it. A period between ".....100pf"
and "the input..." there would have been quite helpful. ;-)

the negative part of the input impedance must be stronger than the
loss in the tuned circuit.
this is affected by the ratio of the 2 100pf capacitors.

So they do act like a voltage divider of sorts and shunt some of the
oscillator output to ground and some back to the input.

a crystal can apear to be a very high inductance at resonance,
at the point where you want to operate it probably has very high
inductance indeed.
you can determnine the eqv inductance by using the equivalent internal
inductance and capacitance. you need to find the mutual capacitance
of the crystal wich is hard to find man specs wich tel you this but
it is often something like 14ff for example.
(0.014pf) you can then work out the eqv series inductance for it to
resonate with the std load wich may be 20pf.

you can then work out what inductance the crystal will apear to have
at the frequency you want. and hence the series capacitance you need.
you might find the inductance is so high that you need less than 1pf
or it has become capacitive.

Cool, a way to figure out just how high you can pull it and how to attain a
certain frequency. I'll probably stick to tinkering though. ;-) Thanks
allot for the detailed explanation. :-)

Arv wrote:
On 14 Mar, 07:42, "Anthony Fremont" wrote:
Anthony Fremont wrote:
Doodling with reactance formulas, it appears that 20uH
(coincidence?) would offset 100pF of capacitance fairly well by
having a an opposing reactance (well resistance at this point) of
about 450 Ohms at

s/.well resistance at this point.//
It's just inductive reactance, I need more coffee. ;-)

3581kHz, the same as 100pF. I'll try putting the coil I made in
parallel and see what happens. Hopefully it won't short the
oscillator and kill my 15 year old NE602, I only have two spares.

Should I be afraid to do this? Does it need something to block DC
current?

Anthony

Your trimmer cap will block any DC flow to ground. If you are really
paranoid then put a 0.1 mfd in series with the crystal.

Doh, I see that now. For some reason I was thinking that they crystal had
both pins connected to the oscillator on the 602, and not with one leg
grounded as the circuit has it. Having yet another senior moment I guess.
Only about the tenth one so far this week.

If you do accidentally ground the crystal input terminal on that
SA602, it only pulls the base of a transistor to ground and thus turns
it off. That shouldn't hurt anything.

While you may not be able to purchase new "602's" now (they are long
ago declared obsolete) the Phillips SA-612 is the same unit and is
readily available from a number of outlets.

I saw that 612 part when I was poking around on the net. A fellow ham gave
me three NE602s about 15 years ago. I had fogotten about them and recently
found them. I even have a couple of tuning cap vernier drives he gave me.

A datasheet for this device is located at:

http://www.nxp.com/pip/SA612AD_01.html

Thanks

Arv - K7HKL
_._

_.. . _. ..... __._ __ __.

On Mar 14, 3:33 pm, "Anthony Fremont" wrote:

what freq you need ?

3581.5kHz to zero beat with the desired signal.

But why do you want to zero beat?

"Anthony Fremont" wrote in message
...
colin wrote:
"Anthony Fremont" wrote in message
...
colin wrote:

Its only a pull of ~100ppm, this should be easily pullable for most
fundamental xtals.

I'd like to get close to 500ppm if possible.

do you need that much ?

I "need" an increase of about .0546%. Isn't that about 546ppm?

what freq you need ?

3581.5kHz to zero beat with the desired signal.

whats the colourburst crystal freq ?

3579.545kHz.

aha ok, thats a fair bit, but maybe within range, as as been said at a
certain frequency the crystal becomes a complete open circuit.



ofc if the tolerenace all add up against you you might find it hard.

I would also try reduce the 100pf caps on the sa602 too.
100pf is higher than most crystals I use would like,
50pf or 20pf or if youve got some spare trimmers ...
you can adjust the ratio too, say just reduce the one accross pin
6-7

I figured that they were voltage dividers to set the amount of
feedback, but I can certainly see how they could have an effect. Since
I'm close to where I need to be, I will try a couple of 33pF
caps to see what happens.

no they are involved in setting the frequency too,
in order for you circuit to work it needs to resonate,
with the 2 100pf the input where the crystal is looks like a
capacitor with some negative impedance,
the circuit with your crystal, inductor, and trimmer must be
inductive, it then forms a resonant ciruit with the capacitance of the
input.
usualy the crystal would just be operated so that it looks inductive.

Ok, I had to read that a few times to get it. A period between
".....100pf" and "the input..." there would have been quite helpful. ;-)

Yeah I kinda got lost in my own explanation myself. didnt have much time to
explain.


the negative part of the input impedance must be stronger than the
loss in the tuned circuit.
this is affected by the ratio of the 2 100pf capacitors.

So they do act like a voltage divider of sorts and shunt some of the
oscillator output to ground and some back to the input.

this circuit is an emiter folower wich has no voltage gain,
so actually they operate in the opposite way wich is kinda confusing but let
me explain ...
consider a typical tuned circuit with LC and a tap in the L,
driving the circuit at the tap acts as a step up,
but the crystal is the inductor wich make it difficult to put a tap here,
but at resonance the 2 capacitors can act in the same way and provide a
voltage step up.

but you can see there is a curent loop involving all these components in
series,
this is what sets the frequency.


If you decrease the capacitor accross the 2 pins of the ic this will
increase the voltage accros it and so give more drive, as wel as increase
the frequency.

another way to look at it is if you consider that ground is at the base then
the capacitors are in fact a voltage divider wich feed into the emiter of a
comon base amplifier.

a crystal can apear to be a very high inductance at resonance,
at the point where you want to operate it probably has very high
inductance indeed.
you can determnine the eqv inductance by using the equivalent internal
inductance and capacitance. you need to find the mutual capacitance
of the crystal wich is hard to find man specs wich tel you this but
it is often something like 14ff for example.
(0.014pf) you can then work out the eqv series inductance for it to
resonate with the std load wich may be 20pf.

you can then work out what inductance the crystal will apear to have
at the frequency you want. and hence the series capacitance you need.
you might find the inductance is so high that you need less than 1pf
or it has become capacitive.

Cool, a way to figure out just how high you can pull it and how to attain
a certain frequency. I'll probably stick to tinkering though. ;-)
Thanks allot for the detailed explanation. :-)

The pulling range is usually equal to the motional capcitance over the
crystal self capacitance, so for 14ff and 20pf this gives 700ppm, but
actually at this extreme its unusable in this circuit as its required to be
inductive.

as said by some1 else the farther you pull it the worse the performance.

Colin =^.^=

wrote:
On Mar 13, 12:02 pm, "Anthony Fremont" wrote:

I believe this is direct conversion and that the crystal should
exactly match the desired station (3581kHz) I want to recieve,
that's right isn't it? Maybe this is close enough? I have to wait
til tonight to see if I can actually hear anything.

Depends on what sort of signal you are trying to receive.

To receive a single our double sideband signal (or, in a very touchy
way, an AM one), you want your direct conversion receiver's local
oscillator exactly on the transmitter frequency.

But to receive a CW signal, you want your local oscillator 700 Hz or
so above or below the transmit frequency. That will cause you to hear
a 700 Hz audio tone for the CW.

Unless I'm mistaken, 3.581 MHz is in the CW portion of the 80 meter
band.

It is, I mistakenly thought they did the 80 meter voice announcement down
there. The voice bulletin is actually on 3990kHz.

Since I have most of what I need (tuning caps, vernier and toroids/wire) I
started looking around at different VFO circuits. I'm trying to find
something that will span from 3500 to 4000kHz and still be fairly simple. I
could then inject this into pin 6 of the 602 and be done with it. :-)

If you can get to 1 KHz below the transmitter, you should be able to
copy CW as 1KHz audio. If you can only get to 1.5 KHz below, you'll
get 1.5 KHz audio... not ideal listening, but probably workable.

Right. I was hoping to be able to "get on the other side" of the signal to
have a second chance at avoiding possible QRM.

wrote:
On Mar 14, 3:33 pm, "Anthony Fremont" wrote:

what freq you need ?

3581.5kHz to zero beat with the desired signal.

But why do you want to zero beat?

For CW I don't, but I would want to be able to get on the other side for QRM
reasons.

On Mar 14, 4:23 pm, "Anthony Fremont" wrote:

If you can get to 1 KHz below the transmitter, you should be able to
copy CW as 1KHz audio. If you can only get to 1.5 KHz below, you'll
get 1.5 KHz audio... not ideal listening, but probably workable.

Right. I was hoping to be able to "get on the other side" of the signal to
have a second chance at avoiding possible QRM.

My guess is the project's original author hadn't planned on that...
most likely, they'd planned on rubbering the crystal only far enough
to get a reasonable audio tone from the RF-LO combination.

Obviously you can get more range out of a VFO, though building nice
VFO's isn't simple.

Even the tuning caps can be a pain. I spent a while as a teenager
knocking alternate plates out of old 365 pf AM broadcast caps to try
to make some suitable for 40 meters - or you can use a series
capacitor. Beware hand capacitance when you go to tune it. Today,
varacter tuning is another option - stable regulated supply and a
multi-turn pot.

Another thing you might do is google the "poundshop" (dollar store)
receiver projects. Those are little KHz-IF varactor tuned auto-
scanning FM radios that people have been modifying into direct
conversion ham band receivers.

On Wed, 14 Mar 2007 04:58:52 -0500, Anthony Fremont wrote:

I guess I'm at the limits of that particular crystal.

I think you are getting close to the limits :-) You might get a
few Hz more by reducing the value of the 2 100pF capacitors. Try
another crystal or a ceramic resonator.

73, Ed. EI9GQ.

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