Receiver Incremental Tuning Crystal Oscillators
The NorCal 40A includes a receiver incremental tuning (RIT)
circuit to offset your receiver frequency from your transmitter
frequency.Why include this feature? Because other transceivers may be operating with receivers and transmitters that aren’t perfectly aligned (in frequency). Or, perhaps their transmitter has drifted in frequency due to heating while their receiver has not.With an RIT, you can finely adjust your receiver frequency
without affecting the transmitter frequency. To operate the RIT:
1. Adjust your VFO so your transmitted signal is being
received by the other station.
2. Next, adjust your RIT so your receiver matches his
transmitter frequency. Cool!
Using the RIT helps avoid a “dog-chasing-his-tail occurrence”
in which station 2 adjusts his transceiver to receive 1 properly
but also changes his transmit frequency. Then station 1 adjusts
his transceiver for reception, but changes his transmit frequency,
and so on. In Prob. 26, the VFO tune pot (R17) is connected to ground through R15 as shown in
Later towards the end of Prob. 27, R15 is removed from ground
and connected to pin 7 of U6, which is the LM393N dual
comparator, as shown in This forms the RIT:
How does this RIT work? It’s actually very simple. The
comparator is similar to an op amp except with an “opencollector” output as shown in
Referring to Fig. 11.18(c), the output pin of the comparator is a
BJT collector terminal. This pin is either open- or short-circuited
to ground depending on Vi and Vr, as:
• Vi >Vr , then Vb is low and V is open circuited (“off”), or
• Vir , then Vb is
high and V is short circuited (“on”).
There are two situations important to us in the NorCal 40A. In
both, we’ll assume that the RIT (S2) is “on”:
1. With “8V TX” high, then the left comparator in Fig. 11.20 is
“off” (low) and the right comparator is “on.” This implies that
the VFO is connected through R15 to ground, and R16 is
effectively open circuited. This is the standard transmit
configuration studied in Prob. 26.
2. With “8V TX” low, then the left comparator is “on” and the
right comparator is “off.” Therefore, the VFO is connected
through R16 to ground and R15 is effectively open circuited.
Now, by changing R16 we can vary the bias voltage of the
varactor D8 and hence the receive frequency (remember that
“8V TX” is low when receiving) and only affect the receiver
frequency. This is called receiver incremental tuning. Crystal Oscillators
Besides the VFO, there are two other oscillators in the NorCal
40A. These are the Beat Frequency Oscillator (BFO) and the
Transmit Oscillator (TO). Both are crystal oscillators. These use a BJT inside the SA602AN IC as the amplifier plus an external quartz crystal and a voltage divider network to make the feedback network. Together, these form a crystal oscillator, as shown in
We can recognize the Clapp oscillator topology in this figure by
the capacitive divider/feedback network. This oscillator is very
similar to the JFET Clapp oscillator used in the VFO. The small signal model for this oscillator is shown Here, we are using a transconductance model for the BJT:
To determine the approximate resonant frequency, we note that
the source-free, lossless circuit that the inductor “sees” is
Therefore
Where
The startup condition for this oscillator is derived in the text to
be:
In this expression, Ib is the base bias current and Vt is the thermal voltage (? 25 mV at room temperature). The startup condition in is a rather complicated expression largely because of the many sources of loss in the circuit. Lastly, the peak-to-peak output voltage is derived in the text as
where R 1= Rb ||Ra and I0 is the dc component of the emitter current.
We see in this expression that with C2 >>C1 , then Vpp ? C1 . This effect will be quite noticeable as you adjust:
1. C17 in the BFO (C17 is “C1” for the BFO),
2. C34 in the Transmit Oscillator (C34 is “C1” for the TO).
The Transmit Oscillator has an additional series inductor L5 that
is used to shift the oscillator frequency to a value different from
the BFO. What purpose does this serve in the radio?
