[Collins] Interesting snippet on today's Collins - MiniCircuits Labs

[Dr. Gerald N. Johnson]

I added a MAV11 the +17 dBm out MMIC between the balanced mixer and the 
crystal filter in the 220 module in the FT-736 hoping to improve channel 
13 intermod. I was unable to detect any improvement in intermod, but the 
added 11 dB gain made it sound like a receiver without having to run the 
audio gain full tilt and them have my ears blown out by a signal 30 dB 
above the MDS.

There often is benefit to a tuned preamp to help the image and harmonic 
rejection of a receiver front end but a double balanced mixer needs 
broad band terminations on at least two of the three ports to give its 
best conversion loss and intermod performance. Reflecting the unwanted 
output back to the mixer (whether that's the other output sum/difference 
or a strong signal just outside the following IF filter) increases the 
signal levels in the mixer diodes and can cause a 30 dB deduction in the 
top end dynamic range.

73, Jerry, Technical Adviser to the Collins Radio Association.

On 4/28/2010 6:43 AM, Carl wrote:
> Those devices then amplify everything from DC to daylight and in an old
> single conversion 455kc radio that means lots of images.
>
> Ive built a few for scope and counter preamps and even to pep up a
> vintage 141T spectrum analyzer.
>
> DEMI used a MAR in their 6M transverter as a post mixer amp. While the
> IM specs look good on paper they fall apart with a real world band
> opening or contest; I wound up removing and bypassing it and feeding a
> bullet proof 2N5109 with a 1.5dB NF at 28 mHz and throttled back to 10dB
> of gain after the BPF.
>
> Carl
> KM1H
>
>