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RF-2800/RF-2900 Frequency Counter Circuit
4
|. Outline:
The RF-2800/RF-2900 displays the frequency of the received broadcast by counting the frequency of the local
oscillator and scaling accordingly. Figure 1 is a block diagram for the receiver. The following relationships exist
between the reception frequency (fs), the local oscillator frequency (fo), the intermediate frequency (fi) and the
display frequency (fo).
(A) fs+fi=fo...(1)
. (B) fo=fs=fotfi... (2)
LOCAL
OSC
. , CONVERSION
COMMAND
DEVICE
Fig. 1
With reference to figure 1, for example
Reception frequency (fs)=10 MHz
Intermediate frequency (fi)=455 kHz
Under these conditions, the local oscillator frequency (fo) must be, according to formula (1), 10.455 MHz (called
"upper local oscillation'') or 9.545 MHz (called "lower local oscillation''). Thus, if 10.455 MHz is used as the local
oscillator signal:
Display frequency (fo)= 10.455 (fo) — 0.455 (f)= 10 MHz (fs)...... (3)
Therefore, the display frequency is equivalent to the reception frequency.
The subtraction of the 0.455 MHz (fi) is accomplished by the conversion command device to the counter.
if the unit were designed to use the lower local oscillator frequency, a signal (preset frequency= + 0.455 MHz)
would be applied to the counter in order to add 0.455 MHz.
At the same time, in models which use different frequency, such as 2 MHz, for the intermediate frequency (fi), a
conversion signal is applied to the counter in order to add (or subtract) 2 MHz, so that the reception frequency will
be correctly displayed.
In short, the conversion signal must be equal to +fi. -
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