John Byrns wrote:

In article , Fred Nachbaur
wrote:

Patrick Turner wrote:

John Byrns wrote:

In article , "John Walton"
wrote:

If you don't go this route, the most difficult item to find for a super-het
will be the ganged 365uF (they called 'em 365mmF) capacitor and the IF
transformers.

Doesn't a TRF receiver also require a "ganged 365uF capacitor"?


For TRF, 4 single tuned circuits are required, so that means two
double gang tuning caps, and then you have to get them all to track
properly. The Q of each tuning circuit dosn't want to be too high, or else
the final BW will be so narrow that so will the AF BW.

The bigger problem is that the Q changes across the band, tending to be
higher at the top end because of the higher reactance (and therefore
higher ratio of reactance to resistance). So there'a compromise between
good sound but lousy selectivity at the low end, and good selectivity
but muddy sound at the high end. The AM broadcast band is bad that way,
since it's about a 3:1 frequency range.

When you speak of the "low end", and the "high end", are you talking about
wavelength, or frequency?

Actually you want the Q to vary across the band, you want the Q to be
proportional to frequency, so that you can maintain a constant bandwidth
as you tune from low frequencies to high frequencies. This effect can be
approximated by making as many as possible of the lossy elements in the
tuned circuits, series elements. This means the tuned circuit should be
driven from a pentode with a high output resistance, and care should be
taken in the design and layout of the amplifiers to make the grid
conductance as low as possible. series resistors can then be added to the
tuned circuits to set the desired circuit Q. Double tuned circuits, using
two tuning condenser gangs for each stage, can also be used to improve the
filter shape factor and achieve steeper skirts without excessive narrowing
of the nose response. Complex coupling impedances can also be used to
maintain the desired coupling as you tune across the band, and also to
provide increased coupling at the low frequency end of the band to help
offset the effects of the remaining parallel losses in the circuit. These
techniques can greatly reduce the compromises necessary in TRF AM
broadcast receivers. The famous Western Electric No. 10A Radio Receiver
is an example of this type of design. J.W Miller also offers a series of
radios and components based on this design technique, and the J.W. Miller
design was used in Altec's early AM tuners.

They invented the superhet to overcome the TRF troubles.

Indeed. Only one frequency to worry about, as far as the selectivity vs.
bandwidth situation is concerned. Also makes it practical to do
stagger-tuning to give a more square-shouldered bandpass characteristic.

Why even bother to stagger tune? Double tuned IF transformers, and even
higher order filters, like the fourth order filters used in some of the
H.H. Scott AM tuners, can accomplish much the same thing by judicious
choice of Qs and coupling coefficients.

Regards,

John Byrns

Interesting.

I have two tuned LC input circuits in the front end of my superhet reciever.
they are loosely coupled to the antenna.
I found that the BW was smallest at the lowest RF and there was significant side
band cutting
and would have been reduction of audio bandwidth, had I not used two LC coils, tuned

slightly apart, to give a wide enough BW.

It is necessary to have at least 60 dB attenuation when 40 kHz off the wanted F,
and 80 to 100 dB is better, and a couple of tuned circuit don't give enough
attenuation away from the wanted F, even if the LC circuit has a high Q.
If a single 455 kHz IF LC circuit has a BW of 20 kHz,
then the Q is only about 22.7.
When 4 such circuits are arrayed, the resultant Q is a lot more, and BW is a lot
less.
Such LC circuits have only say 15 dB attenuation at say 40 kHz off the wanted F,
so you need a few LC circuits to get stations 40 kHz away from the one you want to
be inaudible when there is silence in the programme.
We have a sports station of 300 watts at 1008 kHz, and
there is an entertainment station of 10,000 watts KHz less than a Km away,
at 1,053 kHz, and I know a radio is OK if I can tune to 1017,
and not hear the 1053 in the bacground of 1008 which
will be noisy, and distorted.

Patrick Turner.


Surf my web pages at, http://users.rcn.com/jbyrns/