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Author Topic: BFO's & superheterodyne sensitivity?  (Read 2181 times)

Posts: 225


« on: November 09, 2007, 08:26:50 AM »

Hello Elmers!

Say, can some one explain something about superhet
receivers and the Beat Frequency Oscillator:

(I referring to the 80m AM band here ..)

I understand why one needs a BFO for receiving CW
and SSB communications.  (simply: for SSB, it basically
replaces the “missing” carrier, since only the side-band
information is transmitted.  For CW, it provides a frequency
to beat against so that one can hear an audible oscillation
whenever the sender’s transmitter is keyed.)

But, here’s what I don’t get.  It seems to me (at least
with my vacuum tube equipment) that when the BFO
is turned on, the receiver sensitivity greatly increases.

Besides hearing numerous transmissions that were inaudible
with the BFO turned off, I can hear the noise floor come up,
along with background sputtering, etc.

So, is this just a perception illusion, or does the sensitivity
of the receiver actually change in some fashion with the
BFO turned on .. if so, why?

Hey, Thanks everyone!


Posts: 820

« Reply #1 on: November 09, 2007, 09:42:03 AM » the receivers your talking about when the BFO is turned on the AVC is disabled. The receiver rf and if stages are at their max sensitive. Todays radios using a product detector allows the AVC/AGC to function. Just ride the RF gain control for a level that you like.


Posts: 820

« Reply #2 on: November 09, 2007, 09:58:28 AM »

Just a bit more....I bet that receiver has a AVC off position which is used for weak signal reception. Your noise floor will come up too when using the AM mode and no AVC. Just subject to overload and distortion on a strong signal until you back off the RF gain control.


Posts: 9749


« Reply #3 on: November 10, 2007, 04:26:34 AM »

Many or most receivers do disable AVC when the BFO is on, but not all old receivers disable AGC!!

Even if the AVC is not disabled the addition of BFO injection can increase the low signal level sensitivity.

This is because the BFO "switches" the detector off and on. Instead of a signal having to reach the level where the detector diode conducts (it's non-linear and falls out of conductor at low signal levels because of the conduction threshold voltage of the detector), the injection causes it to switch.

This causes the detector to be more sensitive and more linear to very low level signals.

73 Tom

Posts: 1041

« Reply #4 on: November 10, 2007, 07:54:07 AM »

I know of no receiver that turns the AGC off when
the BFO is turned on and vice versa. What receiver
are you talking about that does this?


Posts: 820

« Reply #5 on: November 10, 2007, 12:37:29 PM »

"What receiver
are you talking about that does this?

Hallicrafters S-40B, Hammarlund HQ-129x are two that I had.


Posts: 9749


« Reply #6 on: November 10, 2007, 05:15:11 PM »

Most receivers that did not have a balanced demodulator or product detector used a diode detector. Many or MOST of them picked the AVC voltage off from that detector.

When the receiver picks the AVC voltage off from the detector the BFO injection will totally saturate the AVC. For that reason many, but not all, older receivers turned off the AVC automatically when the BFO came on.

The general rule is if the AVC was detected from a diode detector that also had BFO injection the receiver would shut off AVC.

If the receiver detected AGC from an earlier stage or later state (audio derived AGC) rather than from the detector, many times the AGC would function on SSB and CW and the AGC was not turned off.

My SX-101 for example can use the AGC/AVC while the BFO is on, so it does not turn the AVC off. The same for my HQ180 and my NC-300 and NC-303 receivers.

On the other hand my Gonset G66, much older Hammerlunds, and older Nationals get AVC from the same diode that detects AM, and so when the BFO injection is added they would go dead. So when the BFO switch on them is turned on the AVC automatically shuts off.

It actually depends on the particular model how they worked it. So if you mostly had one type you would have one opinion. If you had the other type you would have the other opinion.

73 Tom


Posts: 225


« Reply #7 on: November 10, 2007, 06:34:19 PM »

Hello again everyone, and thank you for all your
thoughtful replies!

I guess that I should have explained more about the
circuit that I'm working with.

It's an early 1940's design for a 7-tube super heterodyne
receiver from the ARRL handbook.

The operator has full manual control of the a.v.c.
circuit and BFO.

Here's a link to the schematic, and a bit of explanation
about the how the a.v.c. operates.

But, the phenomenon that I was referring to
(i.e., increased sensitivity when the BFO is turned
on) occurs weather the a.v.c. is on or off.

From the 1941 ARRL Handbook:

The second diode plate is coupled to the fist (the detector diode) through C22.  R18 is the a.v.c. load resistor and R9 the filter resistor.  C12 and C14 are the by-pass condensers (and part of the time constant circuit) for the grid circuits of the i.f. and mixer tube, respectively, the a.v.c. voltage being fed in series with the tank coils in these circuits.  S2 grounds the a.v.c. line to make the a.v.c. inoperative.

Thanks again,

--Tom Nickel KC9KCEP

Posts: 1041

« Reply #8 on: November 10, 2007, 11:46:44 PM »

The Hallicrafters SX-40B and Hammarlund HQ-129x
have separate AVC and BFO switches, neither control
the other. That means the AVC switch has no affect
on the BFO and vice versa.

A receiver's gain is controlled by 2 methods.

1. A variable bias controlled by the RF gain control
that is applied to the receiver's RF amplifier's grid.

2. A variable AGC voltage applied to the receiver's
RF and IF amplifier grids.

Both voltages are negative and each operate
independent of one another. The RF gain control
is the "manual" gain control where the AGC is
the "automatic" gain control. The RF gain control
is based on a bias voltage, not connected to the
incoming signal where as the AGC voltage is based
on the incoming signal strength.

Some receivers have a separate IF gain control.
This control, varies the IF amplifier's cathode bias
through a variable resistor, usually in the
first If amplifier's cathode. This control is not
based on the incoming signal. An example of a
receiver with an IF gain control is the Heathkit
Mohawk RX-1.

The receiver's RF gain control varies the RF
amplifier's grid bias. Increasing the bias
decreases the amplifiers gain. Decreasing the
bias, however, increases the amplifier's gain.

The receiver's gain can also be control by a separate
bias voltage that is obtained by rectifying the
output of the last IF amplifier. This voltage is
also negative and applied to the RF and IF amplifier
grids to control the receiver's gain "automatically".
Some AGC voltages are obtained by tapping the
detector. However, it was found to be more efficient
to have a separate AGC diode, therefore, receivers
changed to a separate AGC diode. The AGC diode
is biased to a threshold level by a resistor
in the diode's cathode. If the incoming signal does
not exceed the AGC diodes threshold level, no AGC
voltage is produced and the receiver's gain remains
unchanged. However, if the incoming signal is strong
enough and the AGC diode threshold level is exceeded,
AGC voltage is produced, reducing the gain of the

The BFO is a simple oscillator that produces
a signal close to the receiver's IF frequency
that beats with the output of the last IF
amplifier signal, producing a "beat" note, hence
"Beat Frequency Oscillator". The frequency
can be varied by adjusting a variable
capacitor across the oscillators tanks circuit,
much like a VFO only limited in the frequency
range, 1Khz or less either side of the BFO's

The basic detector recovers modulation (audio) from
the incoming signal. There are various types of
detectors but the simplest is a diode.

When a BFO signal is injected into the detector,
the detector now becomes what is called as a
"Heterodyne Mixer". The Heterodyne mixer, mixes
the BFO signal with the output signal of the
last IF amplifier producing a beat note. The
mixing and detection process takes place within
the Heterodyne Mixer and the recovered modulation
(audio) is then fed to the receiver's audio amplifier.

A "receiver" does not contain or use a balance
modulator. A balance modulator is used in a single
sideband transmitter/transceiver. The balance
modulator plays no part in the "receiver" portion
of a transceiver, only the transmitter portion.

The BFO and detector play no part in the receiver's
gain or sensitivity. An over zealous BFO only
reduces the ability of the detector to recover
the modulation (audio). This is rare but can
occur due to a failed component in the BFO
or detector circuit. This also can occur in
poorly designed BFO and detector circuits where
the BFO signal level is too high. The normal BFO
injection signal levels are between 5 and 20 times
the incoming signal level. This level is to elminate
or reduce the distortion of the recovered audio.

Technology has changed over the years and today's
receivers have vastly improved circuits. Product
Detectors have replaced BFO and more advanced AGC
circuits provide better receiver gain control. Many
other advances also have been incorporated in modern
day receivers such as DSP filtering, etc. Although
most of the modern day equipment have much more
advanced circuits, additional features not found in
older vintage equipment, the basic concept is the



Posts: 225


« Reply #9 on: November 11, 2007, 05:20:00 AM »

Wow, what a great amount of informaion!  Thanks
again, everyone for all your replies!

But, I don’t think that I have articulated my initial
question clearly :-(

I believe that I have a fairly solid grasp of the
technologies of the various circuits in a heterodyne
receiver (such as I.F. gain, a.v.c, etc ..)

But .. here’s what I’m experiencing.

When my BFO is off, and I sweep the band spread
capacitor, I hear *very little of anything* in terms of background noise, sputter, phone transmissions, etc.

But, when I flip the BFO “on”, it seems as if the
band comes alive.

The clutter & background noise become easily audible.  Many SSB transmissions that weren’t even
audibly perceivable become easily apparent.

It *seems* as if the sensitivity of the receiver has
improved, but I do not know what causes this
*perceived* increase in performance.

Perhaps it is just the particular receiver circuit
that I am working with and no one else has experienced

Maybe it’s just the old technology circuitry that is
doing this (1941 ARRL 7-tube receiver) and this just
doesn’t happen in more modern receivers?

Thanks again,


Posts: 820

« Reply #10 on: November 11, 2007, 03:46:26 PM »

"The Hallicrafters SX-40B and Hammarlund HQ-129x
have separate AVC and BFO switches"

By golly your right Mike, it has been many years since I owned them, but I seem to remember the S-40B did have a toggle switch for those. The HQ-129X used a rotary 3 position switch. AVC off, AVC on and BFO in that order. Once the switch was in the BFO position it turned the AVC off.

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