In the late 60s I built a VLF upconverter using a ring mixer.
I tried a few different devices for the diodes.
The base/collector junctions of germanium switching transistors
gave the best results.
On 09/09/2012 03:21 PM, Bruce Griffiths wrote:
NIST have indicated that mixer PN noise measurements with a non
dissipative terminations (even RF) are intended to be made.
Using a discrete mixer using diode connected transistors may also be
useful at least for 5MHz and 10MHz input frequencies in that their
flicker noise can be significantly lower than for mixers using diodes.
NIST used a simple diplexer arrangement that terminates the RF sum
frequency in 50 ohms whilst using a reactive termination for the
difference frequency.
With such a termination the minicircuits phase detectors had lower
measured PN noise than the 10514.
Another issue to consider is saturated or unsaturated (linear)
operation of the mixer.
Saturated operation, for most mixers, produces lower noise (flicker
and floor) however linear operation can have for some mixers lower
phase shift tempco which can be important depending on the environment
and required phase shift stability.
Bruce
Bob Camp wrote:
Hi
In general, you terminate the mixer in 50 ohms at the RF frequencies
(say 10 and 20 MHz). Termination at the "IF" (in this case audio)
frequencies is what turns out to be tricky. Any time you terminate a
source in a high impedance, you get a higher output voltage.
Reactance rarely adds noise to a termination. With a DMTD, slew rate
is the issue, generally mixer noise goes up less than the slew rate
increases with a "non-50 ohm" termination.
Bob
On Sep 9, 2012, at 4:41 PM, Bruce
Griffiths<[email protected]> wrote:
David Kirkby wrote:
On 9 September 2012 18:28, Pascual Arbona<[email protected]>
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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Developer of Industrial ZMODEM(Tm) for Embedded Applications
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