Mixer local oscillator input levels

sm0vpo wrote:... One point I did notice with the triode/heptode converter is that it is quite noisy. I built the receiver in stages and noticed the rise in noise. ...

Hi all,

heptodes are rather noisy, namely above 10 MHz. Maybe a pentode mixer would be better?

VBR from Ivan

Hi Harry,

The receiver is going now but I'm not sure it's going as good as it should be. Regards your comment "One point I did notice with the triode/heptode converter is that it is quite noisy." Mine has quite a low level of AF output noise which makes me wonder if it's a bit low on sensitivity. However at night the band I'm on comes alive (Chinese, Italian, Spanish broadcasts etc) albeit very little noise off station. I had no noise at all initially but that turned out to be my IF going into oscillation; so some reading up in my ARRL/RSGB manuals suggested neutralisation which fixed the problem. There are no other areas with such issues now. I can't say what the input sensitivity is as I don't yet have a RF test set.

Cheers.

zl1tet wrote:HI Ivan,

Appreciate the reply. Would 1.5-2v p-p on the grid of the mixer be OK for the 'All Valve Receiver'?

Thanks.

As Ivan quite rightly pointed out, the level into any mixer must be a larger "order of magnitude" than the maximum expected RF signal you need to convert.

One point I did notice with the triode/heptode converter is that it is quite noisy. I built the receiver in stages and noticed the rise in noise. So keeping the signal and oscillator levels as high as possible into the mixer will help. I had thought about using a classical diode ring mixer, but my goal was to build the receiver as it would have been built in the 60's. Also the ECH81 gives quite a bit of conversion gain, but a ring mixer would give a loss.

Just a little comment. Best regards - Harry - sm0vpo

Hi Ed,

I am sorry, I really do not have experience with tubes. Try to find typical parameters or characteristics of the tube. But 2V p-p looks quite good.

BTW, "mixer stage is based upon an ECH81 (6AJ8) triode/pentode valve" is a mistake: ECH81 is a triode/heptode or pentagrid. You probably have already mentioned it, too.


VBR from Ivan

HI Ivan,

Appreciate the reply. Would 1.5-2v p-p on the grid of the mixer be OK for the 'All Valve Receiver'?

Thanks.

Hi Ed,

I expect it much depends of the type of the mixer. E.g. a diode ring modulator requires the local oscillator signal so big that the diodes open and close - hundreds of millivolts. A dual gate MOSFET will do with much less signal. Generally the LO signal should be stronger than the input one. Even a clipped LO signal is O.K. with some mixers like a Gilbert cell with bipolar transistors.

VBR from Ivan

Hi All,

For a long time I have not found a definitive answer regarding the level of local oscillator signal required at the input of a mixer. I have a number of ARRL and RSGB handbooks but this simple answer is difficult to ascertain. Should the level into the mixer from the local oscillator be just a 'sniff' or a moderately strong signal? Please explain why.

I've finished my All Valve Receiver and would also like to know how much local oscillator signal (Vp-p) is supposed to be on the grid of the EF80. 

Thanks.

ZL1TET Ed