On Thu, 24 Feb 2005 11:05:46 GMT, "Verizon News"
wrote:

Keepthis in mind:
+7dbm (the recommended RMS input for a level 7 mixer) is about 1.5Vpp sine.
This holds up to reason, as a DBM has two diode-drops that the LO must
overcome in order to switch the diodes properly. Therefore, if you use a
square wave, you also need to supply about 1.5Vpp, or the diodes won't
switch.

There are two common misconceptions many people hold about these devices.
One is that they are inherently 50 ohm devices. In reality you will only see
50 ohms on a given port if the other two ports are also terminated in 50
ohms. You could run the device just as well at 75 ohms if you keep it
balanced properly. Also, many people think that the LO input is power
dependent. This only partially true. It's really voltage dependent,
requiring sufficient drive voltage to turn on the diodes and pump an amount
of current through them that is large compared to the expected input RF
signal.

There is no magic in these things. They're just diode DPDT switches that
reverse the polarity of the RF signal at a rate determined by the LO
frequency. This is essentially the same as multiplying the RF signal by +/-1
at the LO rate. Simple math shows that this multiplication process yields
the sum and difference frequency products. The more symmetrical the LO
switching waveform, the more perfect the multiplication. This is why square
wave LO drive is preferred.

Joe
W3JDR

Thank you. I've worked with them for years and the number of myths
are amazing. An SBL-1 is an amazingly rugged device and +13Dbm
is not a problem.

One problem with using digital logic, especially ttl to drive them is
that TTL is a current and voltage limited output with a variable
output impedence. A logic low can sink more current that a logic high
can source and that means significant non-linear behavour.
Also bipolar TTL logic high is not 5V, typically is around 3.2-3.5V at
rated current load and worse at higher currrent. When you drive a
DBM with it that tends to show as degraded signals, spurs, level
problems. When I drive a DBM with logic I use CMOS drivers as they
offer near symetrical drive for both levels and better overall output
voltage. So it's important to know the driver can supply the current
(about 30ma P-P) and the average 74F74 can do that only marginally and
if there is any attenuation between the '74 and the DBM it's not going
to work. Buffer it with a 74CH244 (octal buffer) with 4 buffers in
parallel and you can easily push the 60maP-P at 5VP-P and allow for a
50ohm 5db attenuator for a more correct level. However, pay attention
to grounding and bypassing the CMOS under those uses they can send
annoying high current spikes into the power sources.

Allison

Verizon News wrote:
Keepthis in mind:
+7dbm (the recommended RMS input for a level 7 mixer) is about 1.5Vpp sine.
This holds up to reason, as a DBM has two diode-drops that the LO must
overcome in order to switch the diodes properly. Therefore, if you use a
square wave, you also need to supply about 1.5Vpp, or the diodes won't
switch.

There are two common misconceptions many people hold about these devices.
One is that they are inherently 50 ohm devices. In reality you will only see
50 ohms on a given port if the other two ports are also terminated in 50
ohms. You could run the device just as well at 75 ohms if you keep it
balanced properly. Also, many people think that the LO input is power
dependent. This only partially true. It's really voltage dependent,
requiring sufficient drive voltage to turn on the diodes and pump an amount
of current through them that is large compared to the expected input RF
signal.

There is no magic in these things. They're just diode DPDT switches that
reverse the polarity of the RF signal at a rate determined by the LO
frequency. This is essentially the same as multiplying the RF signal by +/-1
at the LO rate. Simple math shows that this multiplication process yields
the sum and difference frequency products. The more symmetrical the LO
switching waveform, the more perfect the multiplication. This is why square
wave LO drive is preferred.

Joe
W3JDR



Thank you all for your help, suggestions and especially for the simple
explanation that the mixer diode pairs need enough voltage for
switching. I will remeasure with higher voltages and use buffers that
supply enough current to drive the mixer.

Andreas

Did the audio go through a LPF for the phasing unit?
Remember that a square wave is made up of the sum of many harmonics -
most of these will have been removed and the amplituyde of the
fundamental may not be that large.

Richard


andreas magun wrote:
Hi,

I have assembled a T2 phasing exciter (KK7B) and looked
at its output spectrum with a spectrum analyzer.

When pumping the exciter with quadrature sine wave signals it
works as expected (output power roughly 3 mW, sideband supression
better than 40 db).

However, when pumping with quadrature squarewave
signals of approximately the same power, the output power
drops by a factor of at least 20 db! I would have expected
much more power, similar to that obtained with sine wave pumping.

The quadrature phase signal was generated with a 74F74, the
quadrature signals being attenuated to give a power of approx. 10 dbm
and output impedances of approx. 50 Ohms.

There must be an explanation.

Thanks
Andreas

Thanks for replying.

The problem, was that I had not enough LO-power from the 74F74 outputs.
I have buffered them with a 50 Ohm TTL Driver 74AS804 that could deliver
plenty of current into the LO-inputs. I have limited it to approx 10 mA
by a series resistor of 300 Ohm. At a voltage drop of approximately .550
mVpp across the LO-inputs (approx. 5 mW) the exciter works now as expected.

Andreas

Richard Hosking wrote:
Did the audio go through a LPF for the phasing unit?
Remember that a square wave is made up of the sum of many harmonics -
most of these will have been removed and the amplituyde of the
fundamental may not be that large.

Richard


andreas magun wrote:

Hi,

I have assembled a T2 phasing exciter (KK7B) and looked
at its output spectrum with a spectrum analyzer.

When pumping the exciter with quadrature sine wave signals it
works as expected (output power roughly 3 mW, sideband supression
better than 40 db).

However, when pumping with quadrature squarewave
signals of approximately the same power, the output power
drops by a factor of at least 20 db! I would have expected
much more power, similar to that obtained with sine wave pumping.

The quadrature phase signal was generated with a 74F74, the
quadrature signals being attenuated to give a power of approx. 10 dbm
and output impedances of approx. 50 Ohms.

There must be an explanation.

Thanks
Andreas

On Mon, 07 Mar 2005 17:47:56 +0100, andreas magun
wrote:

Thanks for replying.

The problem, was that I had not enough LO-power from the 74F74 outputs.
I have buffered them with a 50 Ohm TTL Driver 74AS804 that could deliver
plenty of current into the LO-inputs. I have limited it to approx 10 mA
by a series resistor of 300 Ohm. At a voltage drop of approximately .550
mVpp across the LO-inputs (approx. 5 mW) the exciter works now as expected.

That is good to hear. I've had excellent results with T2 using
squarewave drive.

Allison