I recently built two Vectronics direct-conversion HF receivers that
experience some really bad frequency shifts and microphonics when you tune
or handle the radio. Now I understand that direct conversion receivers have
that flaw by nature, but is there any way to reduce it without changing the
entire receiver circuit? Will the matching metal cabinets help much?

Mark W4UDX

If you're starting with a poor design and want to "fix it but don't
change anything", I think you're out of luck. Reincarnation is probably
the only solution. The one thing you might be able to do without major
surgery is to improve the mixer grounding.

The problem is due to LO radiation or other coupling to the front end or
the mixer signal port. You've got volts of LO energy on essentially the
same frequency as nanovolts of signal, and you need to restrict the
mixing of the two to the mixer. It's essential to use a very well
balanced mixer, like a diode ring. Pay careful attention to grounding --
all input and output signals to the mixer should be grounded at a single
point. Shielding of the LO might be necessary. If it's poorly laid out,
the very large circulating currents in the oscillator tank circuit can
flow through the ground system in such a way as to couple back into the
front end. Those problems can be minimized by grounding the bulk of the
oscillator's tank L and C at a single point. Competent layout is the
best solution, but you can sometimes minimize the damage by careful
controlling of the mixer grounds. LO energy escaping out the antenna
connector can cause problems also. This can be minimized by using a well
balanced mixer, properly laid out, and a properly laid out and possibly
shielded LO. An RF stage will help unless the layout is so poor as to
let the signal escape around it, but it's not really necessary.

Direct conversion receivers don't have that flaw by nature. Only poorly
designed or laid out ones do. A well-designed direct conversion rig
isn't any more complicated than a poor one, but the difference in
performance can be remarkable.

Roy Lewallen, W7EL

W4UDX wrote:
I recently built two Vectronics direct-conversion HF receivers that
experience some really bad frequency shifts and microphonics when you tune
or handle the radio. Now I understand that direct conversion receivers have
that flaw by nature, but is there any way to reduce it without changing the
entire receiver circuit? Will the matching metal cabinets help much?

Mark W4UDX

If you're starting with a poor design and want to "fix it but don't
change anything", I think you're out of luck. Reincarnation is probably
the only solution. The one thing you might be able to do without major
surgery is to improve the mixer grounding.

The problem is due to LO radiation or other coupling to the front end or
the mixer signal port. You've got volts of LO energy on essentially the
same frequency as nanovolts of signal, and you need to restrict the
mixing of the two to the mixer. It's essential to use a very well
balanced mixer, like a diode ring. Pay careful attention to grounding --
all input and output signals to the mixer should be grounded at a single
point. Shielding of the LO might be necessary. If it's poorly laid out,
the very large circulating currents in the oscillator tank circuit can
flow through the ground system in such a way as to couple back into the
front end. Those problems can be minimized by grounding the bulk of the
oscillator's tank L and C at a single point. Competent layout is the
best solution, but you can sometimes minimize the damage by careful
controlling of the mixer grounds. LO energy escaping out the antenna
connector can cause problems also. This can be minimized by using a well
balanced mixer, properly laid out, and a properly laid out and possibly
shielded LO. An RF stage will help unless the layout is so poor as to
let the signal escape around it, but it's not really necessary.

Direct conversion receivers don't have that flaw by nature. Only poorly
designed or laid out ones do. A well-designed direct conversion rig
isn't any more complicated than a poor one, but the difference in
performance can be remarkable.

Roy Lewallen, W7EL

W4UDX wrote:
I recently built two Vectronics direct-conversion HF receivers that
experience some really bad frequency shifts and microphonics when you tune
or handle the radio. Now I understand that direct conversion receivers have
that flaw by nature, but is there any way to reduce it without changing the
entire receiver circuit? Will the matching metal cabinets help much?

Mark W4UDX