UHF Receiver schematic and MDS

[darcyrandall2004]

Hello,
Attached is a schematic that now only resembles the UHF receiver I have been designing. It simulates fine but when placed on a board it required alot of modifications to get working.

Anyway I can inject a signal at the Antennae port at -100dbm and view the signal on the spectrum analyzer after the saw filter at -90dbm. The oscillator works fine aswell. Presents about -15dbm to the mixer. The problem is the Mixer. I need to increase the signal strength at the antennae input to -50dbm before I will see a signal at 21.4MHz after the mixer with a signal strength of -90dbm.

Has anyone got an ideas as to what I can investigate to solve this problem?
I expect 10db conversion gain from my mixer. The equation for Minimum detectable signal states that MDS = -174dbm + NF(db) + 10LOG(bandwidth Hz) + SNR(db)
The Saw filter has a bandwidth of 8MHz. Assuming 10db NF my MDS = -94+ SNR(db). Surely I can expect better results than what I have been achieving in reality.

I have even experimented with other mixer designs including a 3V version. Any ideas?

 

[vfone]

To get the maximum conversion gain from a single-transistor additive mixer some particular impedances should be present at mixer ports.

The LO node should be a short circuit at the RF and IF frequencies, while the RF node should be a short circuit at the LO frequency to prevent the LO leakage into the RF port.
Terminating the IF port in a reactance at image frequency can improve also the conversion gain of the mixer.

Easier to get good performances is to use a different kind of mixer, perhaps a dual-gate MOSFET, where are not too many things to care and it's cheap also.

 

[darcyrandall2004]

Cheers Ill will give a dual gate mosfet a try. Thankyou