Mixer local oscillator input levels
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Re: Mixer local oscillator input levels
Hi all,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. ...
heptodes are rather noisy, namely above 10 MHz. Maybe a pentode mixer would be better?
VBR from Ivan
Ivan- Posts : 794
Join date : 2012-11-25
Age : 64
Location : Praha, Czechia
Re: Mixer local oscillator input levels
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.
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- Posts : 7
Join date : 2024-04-04
Re: Mixer local oscillator input levels
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
sm0vpo- Admin
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Join date : 2013-03-26
Age : 72
Location : Märsta, Sweden
Re: Mixer local oscillator input levels
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
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
Ivan- Posts : 794
Join date : 2012-11-25
Age : 64
Location : Praha, Czechia
Re: Mixer local oscillator input levels
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.
Appreciate the reply. Would 1.5-2v p-p on the grid of the mixer be OK for the 'All Valve Receiver'?
Thanks.
zl1tet- Posts : 7
Join date : 2024-04-04
Re: Mixer local oscillator input levels
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
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
Ivan- Posts : 794
Join date : 2012-11-25
Age : 64
Location : Praha, Czechia
Ruud likes this post
Mixer local oscillator input levels
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
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
zl1tet- Posts : 7
Join date : 2024-04-04
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