Local Osc. Question

[JHGibson]

Getting back to the Radiotron Handbook, on page 997, para B it notes that with the LO on g1 and lower in freq than the signal freq, then some negative impedance is generated in the signal circuit which increases its Q. This leads to better image rejection and increased sensitivity.
This is how I read the paragraph.
John.

[MichaelR]

Originally Posted by Skywave

Hi John,
Yes, I read that reference. However, read further down the page. I am totally mystified as to how the author can claim that changing from high-frequency to low-frequency oscillator operation will bring about an improvement in the image rejection ratio by about a factor of 3 and, moreover, that that increase will decrease as the lower end of the shortwave band is approached.
To me, all of that seems to me to be a total and complete contradiction of what can be shown, mathematically and in practice, to be the case.

Al.

Hi Al,

I think the author is coping with the reality of circuit design, he does stress that this statement is true if there are no neutralising techniques employed.He is really talking about practical design and from an era in the relative early days of valves.

MIke

[ms660]

Was up 'till 3.30 am reading bits of the Radiotron book...very intersting, decided today to have a mess about, the SX 28 has a G1 Osc. feed, so on the high bands I tweeked everything way off then proceeded to re align....on band 5 (11 to 21 Mhz) I could only get it to track right when the Osc. was below the signal at the high end of the band and above the signal at the low end of the band.
I have not re aligned band 6 yet but when I do I will post the findings.

Hats off to those early receiver designers as well.

Lawrence.

[ms660]

I re aligned band 6 today on the SX 28, it only tracked properly when the Local Osc. was below the signal.

Band 6. Alignment points...36 Mhz(fs) Osc. low.
22 Mhz(fs) Osc. low.

Band 5. Alignment points...20 Mhz(fs) Osc. low.
11 Mhz(fs) Osc. high.

Osc. high on all bands below the above bands at all alignment points.

So there we go..what was said in the Radiotron book about the osc. below Fs on the higher shortwave frequencies seems to have some relevance for G1 osc. fed pentagrids.

Lawrence.

[MichaelR]

Good work, lawrence

Mike

[ms660]

Yes...a bit of a pain to do because I also did the padders which the manual says never touch, mechanicaly the padders are a bit of a strange arrangement on those receivers, to make it worse most of them were rusted up...anyway all done now.

All the best.

Lawrence.