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