David Kirkby wrote:
On 9 September 2012 18:28, Pascual Arbona<p.arb...@securimar.com> wrote:
Hello Brian,
I am a radio amateur and and also in the Time nuts list, At the
moment I am planning to bild a DMTD for experimentation. and as you have a nice
experience in this field , for me will be very wellcame your help. My ask is
¿whitch is te best temination for the mixer? (FI 10Hz or 100Hz) ¿what about
the amp, limiters and zero crosing detec.)
Mixers should be terminated in 50 Ohms - at least at all frequencies
where there is an output. Someone mentioned minicircuits. They sell
constant impedance filters, where the impedance in the both the
passband and stopband are 50 Ohms. Most filters have a impedance far
away from 50 Ohms in the stopband.
This is an often repeated fallacy.
For low IF frequencies such as in a DMTD, a reactive IF termination can
have the advantage of lower noise and larger IF signal slew rate.
The resultant reduction in RF and LO port VSWR is easily corrected (at
least for low RF and LO frequencies) with a series resistor and/or
resistive pad.
There are a number of NIST papers on the effect of mixer termination for
DMTDs and phase noise measurements.
The minicircuits phase detectors (actually specialised mixers) are
specified for use with 500 ohm IF terminations.
I don't know the details of what you are trying to do, but keep in
mind what I said.
Dave
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Bruce
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