UHF Mixer and Diplexer

[darcyrandall2004]

uhf mixer

Hello

I am making an attempt at designing a pager receiver. The mixer will consist of a single mosfet.

Attached is the current schematic and the simulated results.

Design decisions I have made include:
1. Mosfet to be operated in the Triode region.
2. IF frequency = 21.4MHz, RF = 454MHZ, LO=475.4MHz
3. Diplexer consisting of High pass and Low pass filter with a crossover frequency
of 1.4*21.4MHz ie 30MHz. The Diplexer presents 50ohm to the mosfet across all frequencies.
4. I am guessing that I need to simultaneously provide an impedance match for maximum power transfer at 21.4MHz, 454MHz and 475.4MHz. This is difficult as the different frequencies require a different load and source impedance for max power transfer. I simply chose 50ohms as the terminating impedance. I measured the input impedance at the two gates of the transistor then provided a conjugate impedance match to the 50 ohm sources, the RF input and LO input.

Attached is the results.
I managed to achieve -5dbm conversion gain. NFssb 56.707

Can anyone make any recommendations as to things I can try or improve on this design?
What sort of performance should I expect to be able to achieve with simple design like this?

Thanks





[/img]

 

[VSWR]

uhf diplexer

The circuit diagram details are hard to read. Archive your project files (ZAP or WinRAR) and upload here.

 

[vfone]

bf998 mixer

Using a dual gate BF998 MOSFET as a UHF mixer the conversion gain should be positive, approx +10dB, and the NF just couple of dB's.

Generally the diplexers used after the mixers are in the format presented below.
This is a 50 ohms inp/out diplexer, but the impedances can be changed.

 

[darcyrandall2004]

mixer uhf

How do I determine the most desirable impedance for the diplexer? Obviously I want an impedance that will maximise conversion gain at 21.4MHz but then this means I cant provide an impedance match for max gain at the LO or RF frequencies?

Have you got a basic procedure I should follow when designing this? A decent text book?

Thanks again

 

[vfone]

bf998 mixer design

There is nothing special. At the base of any impedance matching for maximum power transfer stays the rule that the load impedance shall be equal to the source impedance.

Conversion gain of the mixer depends by proper matching of the RF port (for RF frequency), of the IF port (for IF frequency), and depends also by the LO level.

 

[darcyrandall2004]

uhf mixers

At the IF port, yes I have to provide an impedance match that allows for maximum power transfer at the IF frequency. However to prevent the LO signal also present at the output, from reflecting back and worsening distortion I also need to provide an impedance match at the output that allows for maximum power transfer at the LO frequency.

A diplexer will be used to provide the transistor with the desired load impedance. The diplexer will dump the LO frequency at the output into a 50ohm dummy load, likewise the IF frequency will be passed to the next stage.

Providing an impedance match for max power transfer at two different frequencies is proving difficult.

Secondly I determined that if I use a load impedance of 84+324i, I will have a LO power gain of 7db. However this load results in bugger all gain at the IF frequency.

Cheers