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Old 24th Mar 2019, 2:15 pm   #21
Skywave
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Default Re: Function of diodes in superhet radio circuit ?

Whilst not wishing to 'upstage' Sideband's earlier posts and the arithmetic therein, here is my understanding of the oscillator frequencies.

The oscillator will always be on the 'high frequency side' of the received signal.
If we call the osc. freq. Fosc, the required signal freq. Fsig and the intermediate freq. Fif, then we have: Fosc - Fsig = Fif. (Fosc > Fsig) Therefore, Fosc = Fif + Fsig.

Fif is fixed - I'll assume at 455 kHz. Fsig varies: from 1500 kHz to 550 kHz for the M.W. band. Therefore, Fosc must vary as well.
For Fsig = 1500 kHz, Fosc = 1500 + 455 = 1955 kHz.
For Fsig = 550 kHz, Fosc = 550 + 455 = 1005 kHz.
At any intermediate Fsig in the range 550 to 1500 kHz, Fosc. must be of a freq. commensurate with Fosc - 455 kHz (Fif) = Fsig

As a rider to all that, you can see that the freq. ratio of Fosc is 1955 ÷ 1005 = 1.95. However, the freq. ratio of Fsig. is 1500 ÷ 550 = 2.73. So the two ratios are not equal. From a design viewpoint, that causes a bit of a problem. The Fsig frequencies are pre-determined: we cannot change them, nor the ratio. But we can arrange for the osc. frequencies and their ratio to meet the aforesaid requirement, albeit nearly. Doing that usually involves a combination of selecting a suitable capacitance range for the Fosc. variable capacitor, the value of its tuning inductance and the addition of a capacitor in series with the Fosc variable capacitor, usually known as a padding capacitor. Arranging that the equation Fosc - Fif = Fsig is valid over the range of Fsig is known as 'tracking'. The necessary arithmetic / algebra can be a bit involved. The usual compromise is to obtain correct tracking at three Fsig freqs.: usually slightly LF of Fsig HF; slightly HF of Fsig LF and the geometric mean of Fsig HF and Fsig LF.

Again, HTH,
Al.
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Old 24th Mar 2019, 3:34 pm   #22
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Default Re: Function of diodes in superhet radio circuit ?

Going back to the original questions about the diodes, the two that provide a stable power supply at least, you practically never see this method used in 3v powered radios that use germanium transistors, because the initial bias voltages to get the transistor's well into conduction are only around 0.27 to 0.3v and the emitter currents are easily set for the small signal stages with resistors that drop voltages of around 1v. So the divider network can be supplying around 1.3v at the transistor's base. Because of this, the base bias supply resistors are just standard derived from the RF filtered battery supply voltage.
This means that the 3V supply can drop down to as low as 1.5V or even a little less as the batteries go flat and the stages keep running as there is still enough bias voltage for the germaniums.
Trying to make a 3V powered radio with Silicon transistor's where the L/O and IF stages stay working as the battery goes flat is more difficult because of the 0.65V B-E drop of the transistor's. So in your circuit they attempted to ameliorate this issue by stabilising the supply and bias voltages using the two series diodes as a crude shunt voltage regulator. The other diode there is the usual bias diode to help temp-compensate the bias to the output transistor's, especially since they have no emitter resistors.
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Old 25th Mar 2019, 12:51 am   #23
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Default Re: Function of diodes in superhet radio circuit ?

Quote:
Originally Posted by Sideband View Post
Try fitting a 100K resistor in series with one of the meter leads and see if that gives a better reading.
I connected a100K resistor but this completely killed off the frequency reading unfortunately. Using a 10 nF cap instead did allow a reading but there was very little difference compared to the reading without one.
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Old 25th Mar 2019, 1:34 am   #24
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Default Re: Function of diodes in superhet radio circuit ?

Quote:
Originally Posted by Sideband View Post
Hi Al.

Can you correct some errors on my post 14? I've incorrectly said highest frequency of MW was 1500 metres when it should be (approx) 200 metres or 1500Khz. I was obviously thinking frequency rather than metres!


So at the highest frequency of MW (let's call it 200 metres which is 1.5Mhz or 1500Khz) = 1500 + 455 = 1955 Khz which is 1.95Mhz.

so your oscillator should never be running higher than around 1.95Mhz or lower than about 955Khz for MW tuning.

Cheers.
Thanks everyone for the interesting info.
In my earlier post, when I referred to the reading of 2.6 MHz, that was without the tuning capacitor connected. With a suitable polyvaricon connected, the readings are 785 kHz with the polyvaricon fully meshed and 2 MHz with it fully open. So it seems that I am 955-785 = 170 kHz below the correct oscillator frequency required to read the lower end of the MW band, although I'm in the ballpark of the upper end of the MW band.
My further thoughts are maybe to add a padding capacitor in series with the tuning cap (value to be determined by experimentation) to correct the tuning range. Does that sound right ?

Last edited by Jolly 7; 25th Mar 2019 at 1:41 am.
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Old 25th Mar 2019, 7:27 am   #25
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Default Re: Function of diodes in superhet radio circuit ?

Generally, a padder cap is only required when the two tuning gangs on the variable capacitor have the same value. The Polyvaricon (or typical transistor radio tuning cap ) has already had the total pF value of the osc gang, and range of that value (reduced) and made correct so that a padder capacitor is not indicated. The L/O frequency at the low end of the band is adjusted for the correct osc frequency using the red osc coil tuning slug, and at the high end of the band the trimmer cap on the Polyvaricon is used to set the frequency there.

In the early evolution of transistor radios, the designers were using some two and three gang V/C's which were valve radio parts with equal pF values per gang, all needing padder caps for the osc gang. But the Japanese and Philips, quickly started making V/C's with the small osc gang, eliminating the need for padder caps, and that gave much better tracking.

....actually many valve radios would have benefited from a reduced osc pF value on the V/C, but early designers were stuck with the equal pF value V/C's that were a hang over from the days of TRF radios before superhets got invented, and so they needed padder capacitors.

Last edited by Argus25; 25th Mar 2019 at 7:49 am.
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Old 25th Mar 2019, 10:15 am   #26
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Default Re: Function of diodes in superhet radio circuit ?

Thanks for explaining that Hugo. I had often wondered why I hadn't seen a padder cap in some superhet designs. With the inclusion of the LW band, does this still apply to tuning caps that have different capacitance oscillator and aerial sections?

Regards,
Symon.
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Old 25th Mar 2019, 11:01 am   #27
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Default Re: Function of diodes in superhet radio circuit ?

Quote:
Originally Posted by Argus25 View Post
Generally, a padder cap is only required when the two tuning gangs on the variable capacitor have the same value... actually many valve radios would have benefited from a reduced osc pF value on the V/C, but early designers were stuck with the equal pF value V/C's that were a hang over from the days of TRF radios before superhets got invented, and so they needed padder capacitors.
I'd be interested in more info on this!

It's certain that with a 2-gang capacitor with identical sections, a padder capacitor is necessary. However, just making a 2-gang capacitor with scaled-down vanes for the oscillator, so it has smaller total capacitance, isn't enough to throw away the padder. All that would happen is you'd have an oscillator which runs at a proportionally higher frequency throughout the tuning range.

But it isn't a proportionally higher frequency we want. It's a higher frequency by a constant difference, not a constant ratio.

That says to me, that to dispense with the padder, the oscillator vanes would need a different shape to the signal vanes, so they followed different capacitance vs. rotation laws.

Was this done?
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Old 25th Mar 2019, 11:11 am   #28
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Default Re: Function of diodes in superhet radio circuit ?

I've certainly seen 2 gang VCs with different numbers of and differently profiled vanes in early simple MW-only transistor radios, also ISTR there was a Jackson 208 + 176pF (?) unit for just this purpose. Whilst vanes can be profiled for perfect LO-to-RF tracking, it limits a radio to one band of perfection!
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Old 25th Mar 2019, 1:37 pm   #29
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Default Re: Function of diodes in superhet radio circuit ?

Quote:
Originally Posted by turretslug View Post
Whilst vanes can be profiled for perfect LO-to-RF tracking, it limits a radio to one band of perfection!
That's what I was wondering about.

Regards,
Symon.
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Old 25th Mar 2019, 8:16 pm   #30
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Default Re: Function of diodes in superhet radio circuit ?

For the tranny equivalent of a MW only AA5, that would also be a useful cost saving- no padder and less setting up.
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Old 25th Mar 2019, 9:08 pm   #31
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Default Re: Function of diodes in superhet radio circuit ?

With the air variable types, not only were the osc vanes profiled but the side vanes could be adjusted increase or decrease the capacitance over small sections of the tuning range. Generally though as pointed out it is only idealised to one band.

Multi-band radios have always been a problem, either requiring different padder values per band when the V/c has the same capacity per gang (like the EC-10 radio) or by having a multi section Polyvaricon type, typically seen in some Japanese transistor radios that cover short wave.

I have not pulled enough polyvaricons apart to look at the profile of the osc gang, but no doubt is was optimised to reduce tracking errors across the band.

Generally the manufacturer's alignment instructions gave the high and low frequencies to peak the RF/Ant sections on, when the osc was calibrated at the high and low ends of the dial markings. So the tracking was only ever perfect at those two points and somewhere in the middle of the band with equal and opposite small errors on either side.

Part of it was eliminating adjustments and parts. If a radio had a combination of an adjustable core osc coil and a padder cap, it was a disaster as many tuning combinations appeared to work, they became possible due to different settings of the padder and osc coil and only one combination had the minimal tracking errors, but the technician would not know.
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Old 25th Mar 2019, 9:27 pm   #32
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Default Re: Function of diodes in superhet radio circuit ?

Quote:
Originally Posted by Argus25 View Post
Part of it was eliminating adjustments and parts. If a radio had a combination of an adjustable core osc coil and a padder cap, it was a disaster as many tuning combinations appeared to work, they became possible due to different settings of the padder and osc coil and only one combination had the minimal tracking errors, but the technician would not know.
Ooh, I wouldn't be quite so hard on three-point tracking! (Trimmer C, padder C, moveable inductance core) It crops up particularly on better-quality LF receivers with up-conversion, where mistracking between RF and markedly higher frequency (smaller frequency ratio) LO can really rear its head. The correct adjustment protocol is padder towards LF end, inductance core around mid-band and trimmer towards HF end, where error becomes convergent with repeated passes- if an uninformed technician takes the "conventional" two-point tracking adjustment protocol of inductance at LF and trimmer at HF, then error becomes obviously divergent.

Two radios here use this three-point technique on their LF bands, I expect it's pretty common amongst similar class receivers.

Whilst mentioning divergence, apologies to the moderators! I'd wondered if the original 2.6MHz quoted was a result of a counter measuring the second harmonic of a distorted/loaded 1.3MHz LO? 1.3MHz would be credible in the context.
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Old 26th Mar 2019, 12:03 am   #33
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Default Re: Function of diodes in superhet radio circuit ?

Again going back to the original circuit and diodes and bias arrangements, especially of the IF amplifiers, I can see why some constructors would have had trouble getting this radio to work, because the design is too dependent on the hfe of the individual transistor specimens put in there.

In this circuit they chose to stabilise the bias supply voltage then supply a base current from that via a single resistor . But there is very little emitter degeneration (for DC) note the 150 and 51 R resistors in v2 and v3's emitter. So, even though they thought the bias currents would be stable, due to the diode stabilised bias supply, as the battery went flat, the transistor's emitter and collector currents can be widely variable due to the transistor's hfe, especially for v3. It is better to have larger value emitter resistors and take the bias from the usual resistor divider for the transistor base circuits. Maybe it was a lame attempt at saving resistors.
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Old 26th Mar 2019, 3:49 am   #34
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Default Re: Function of diodes in superhet radio circuit ?

Quote:
Originally Posted by turretslug View Post

Ooh, I wouldn't be quite so hard on three-point tracking! (Trimmer C, padder C, moveable inductance core) It crops up particularly on better-quality LF receivers with up-conversion, where mistracking between RF and markedly higher frequency (smaller frequency ratio) LO can really rear its head. The correct adjustment protocol is padder towards LF end, inductance core around mid-band and trimmer towards HF end..
Yes it might be ok if the technician has the instructions where to set the osc core and padder, but if they are both adjustable and the instructions are not there, it can be set in a way where the tracking errors are high, but otherwise looks ok. It is a matter of spreading the errors equal and opposite above and below the centre and at two frequencies near the upper and lower tuning limits (but not on them)
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