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Measuring and selecting the actual received frequency

in regenerative and direct conversion receivers

by SV3ORA

The current article refers to direct conversion receivers with applied regeneration.
Before reading the current article, you should consider this very well written article by
Charles Kitchin N1TEV about regenerative receivers and their advantages. In this page, I
will focus on two main things:

A simple method that could be used for measuring the actual received frequency in a
simple regenerative receiver, such as the one presented by N1TEV.

An operational process that can be used to select the wanted signal and manually
distinguish it from the image one, in such a receiver.

These methods could apply directly into non-regenerative direct conversion receivers too.

To begin, let's consider a basic regenerative receiver shown below. Many radio amateurs
do not really know how a regenerative detector is really tuned in different modes, so I
will start from this, based on information from Wikipedia.

For AM reception, the gain of the regeneration loop is adjusted so it is just below the
level required for oscillation. The result of this, is to increase the gain of the
amplifier by a large factor at the bandpass frequency (resonant frequency), while not
increasing it at other frequencies. So the incoming radio signal is amplified by a large
amount, increasing the receiver's sensitivity to weak signals. The high gain also has the
effect of sharpening the circuit's bandwidth (increasing the Q factor) by an equal
factor, increasing the selectivity of the receiver, its ability to reject interfering
signals at frequencies near the desired station's frequency. For AM signals the tube also
functions as a detector, rectifying the RF signal to recover the audio modulation.

For the reception of CW radiotelegraphy (Morse code) signals, the feedback is increased
above the level of oscillation, so that the amplifier functions as an oscillator (BFO) as
well as an amplifier, generating a steady sine wave signal at the resonant frequency, as
well as amplifying the incoming signal. The tuned circuit is adjusted so that the
oscillator frequency is a little to one side of the signal frequency. The two frequencies
mix in the amplifier, generating a beat frequency signal at the difference between the
two frequencies. This frequency is in the audio range, so it is heard as a steady tone in
the receiver's speaker whenever the station's carrier is present. Morse code is
transmitted by keying the transmitter on and off, producing different length pulses of
carrier ("dots" and "dashes"). The audio tone makes the carrier pulses audible, and they
are heard as "beeps" in the speaker.

For the reception of single-sideband signals, the circuit is also set to oscillate. The
BFO signal is adjusted to one side of the incoming signal, and functions as the
replacement carrier needed to demodulate the signal

But, can regenerative detectors filter out the image signal?

Considering simple regenerative detectors such as the one presented by N1TEV, when
receiving AM, these circuits can distinguish the desired AM signal from the unwanted ones
within the band of interest as they can reject interfering signals at frequencies near
the desired AM station's frequency, but they will pick up both sidebands of the desired

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AM signal. When oscillating, to receive SSB or CW, they will receive both the desired
signal and the unwanted image (in SSB, the opposite sideband). This is because, despite
the fact that they achieve good selectivity due to regeneration, they do not have
anything like enough selectivity to reject the nearby image (opposite unwanted sideband).
Note, this is the general case, but there are some nice designs which have very smooth
regeneration and can effectively set the unwanted sideband out of puff and attenuate it,
so that they can effectively receive only the wanted one. However, in this article I am
talking about regenerative receivers in general.

Whereas in a single conversion superheterodyne receiver (assuming 455KHz IF and no front-

end RF filtering), the image signal is 455KHz apart from the first local oscillator, in a
direct conversion or regenerative receiver the image signal is only a few KHz apart,
nominally 1-3KHz, depended on the mode of operation (CW/SSB).
Usually, in a single-conversion superheterodyne receiver the image is filtered out by
front-end RF filtering that "follows up" the BFO oscillator, so it does not contribute to
the noise, apart from internally generated image noise within the mixer. This internal
noise is still present in an AM detector, so there is no 3dB advantage. Some crude
superheterodyne receivers do not filter or otherwise remove the image, so they suffer a
3dB disadvantage when compared to a better superhet.

Now that you have a good understanding of the above, we can proceed to a simple method
that could be used for measuring the actual received frequency in a simple regenerative
receiver. As you go through the article, you will also find out an operational process
that can be used to select the wanted signal and manually distinguish it from the image
one.

Measuring the actual received frequency

Measuring the exact actual received frequency has a real value only on CW and other
ON/OFF keying modes, or other modes that use single RF frequency for transmission. On SSB
voice mode, the transmitted sideband is a complex function of the audio waveform that has
modulated it, the human voice. There is no single frequency to measure, as there is no
single frequency human voice. Apart from switching the carrier on and off, CW and CDW
could also be transmitted by modulating an ssb signal with a single frequency audio tone.
In this case, the RF frequency that will be produced during each audio tone switching
will be of a single frequency as well and measuring the actual frequency has a real value
too. An AM signal is composed of the carrier and the two modulated sidebands, so
measuring the actual frequency has not a real value.

Before starting describing the method that can be used to measure the actual received
frequency an important thing has to be mentioned. There is a confusion between displaying
the frequency of a receiver and measuring the actual received frequency. Many of the
direct conversion receivers just display the receiver's local oscillator frequency. Some
modern superheterodyne receivers are "clever" and they compensate the local oscillator
frequeny by the IF frequency and display a more accurate value. Nevertheless, all these
methods actually fail to measure the actual received frequency accurately.

Even in the "clever" receiver case described above, the actual received frequency cannot
be determined accurately. This is mainly due to the wider bandwidth of the IF filters
used. For example, consider a CW received signal on frequency 1455KHz, the local
oscillator on 1000KHz and the IF filter on 455KHz with 1KHz of bandwidth, with the peak
of the band pass of the filter at 455KHz (ignoring the image for this example). If the
frequency of the incoming RF signal changes a few 10s or even 100s of Hz, the display
frequency of the receiver will still show 1455KHz. The only thing that changes in this
case is the audio tone drift that will be listened, which denotes a change in the
incoming signal frequency, eventhough your receiver display frequency has not been
changed. If the IF filter is narrower the accuracy will be better, but never good enough.
Furthermore, filter responce and local oscillator may be subject to frequency drifting as
temperature changes and this can make the things even worse.

The only method I can think of, which can be used in regenerative and direct conversion
receivers to measure the actual received frequency accuratelly down to the Hz, is
continuously measuring the local oscillator signal frequency as well as the audio tone
signal frequency and compensate for their difference to determine the received RF
frequency.

This method is not affected by frequency drifting due to temperature changes because
since there is only one RF mixer stage, any drifting in the local oscillator will present
a drift in the audio frequency. These drifts will be analogous so their difference will
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always be the same. It is this difference the one that is needed to determine the actual
received RF frequency accuratelly.

Using this method, an ultra stable receiver local oscillator is not needed, to determine
the received frequency. Only accurate frequency counters are needed and these can be
cheaply made with accuracy down to the Hz, using microcontrollers.

This method has a disadvantage though. The drifts of the local oscillator frequency and
the audio frequency are known to be analogous, thus their difference can be calculated
and it will be always the same regardless of the drift. Their difference is known but
what is not known, is where the local oscillator signal lies. It can be below or above
the received signal. If the local oscillator is below the received signal, the difference
has to be added to the local oscillator signal to determine the actual received
signal. If the local oscillator is above the received signal, the difference has to be
subtracted to the local oscillator signal to determine the actual received signal.

There is a method that can be used to determine wheather the local oscillator signal is
below or above the received signal. This method monitors the audio frequency change to
determine the above. To illustrate this, let's consider the next example:

Let's assume that the local oscillator signal is at 100KHz and the RF signal at 101KHz.
The audio signal will then be 1KHz and this will include the wanted 101KHz signal as well
as the 999KHz image signal too. For the time being let's assume that there is no signal
present in the image signal frequency, to make the example easier to illustrate.

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Back in the receiver, the only things known to you, are the local oscillator frequency,
which is 100KHz and the audio frequency signal (i.e. the difference), which is 1KHz. You
do not know if the acual RF signal is at 101KHz or at 999KHz. To determine this, move
your local oscillator frequency a bit on the "left" (lower frequency), say at 99.5KHz. In
our example, the local oscillator will be moved 1.5KHz appart from the RF signal,
producing an audio tone of 1.5KHz.

Now move your local oscillator frequency a bit on the "right" (higher frequency), say at
100.5KHz. In our example, the local oscillator will be moved 0.5KHz appart from the RF
signal, producing an audio tone of 0.5KHz.

The lowering of the tone frequency when increasing the local oscillator frequency, can
give you the indication that the RF signal is above the local oscillator signal. Thus, to
determine the actual received frequency of the RF signal you have to add the difference
(i.e. the audio tone frequency) to the local oscillator frequency.

In the inverse case, the lowering of the tone frequency when decreasing the local
oscillator frequency, can give you the indication that the RF signal is below the local
oscillator signal. Thus, to determine the actual received frequency of the RF signal in
that case you would have to subtract the difference (i.e. the audio tone frequency) from
the local oscillator frequency.

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