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Explanations on the wiring diagram
Most VHF superhet receivers use an intermediate frequency of
10.7 MHz. The receiving frequency is first converted to the in-
termediate frequency and then filtered, amplified and demodu-
lated. The VHF retro radio is a superhet, too, which converts
the received signal to an intermediate frequency. But its inter-
mediate frequency is much lower, at 70 kHz. For this reason the
intermediate frequency filters are able to function without bal-
anced coils. The FM demodulator becomes simpler and better
protected against distortion. All essential stages fit into a single
SMD-IC, the TDA7088 with 16 pins. Instead of a variable ca-
pacitor, as used in older receivers, the radio works with the D1
varactor diode. The higher the voltage at the diode, the lower
is its capacity and the higher the receiving frequency. The only
balance point is the L1 coil which can be used to adjust the
lower edge of the oscillator frequency.
The low frequency power amplifier is a simple class-A ampli-
fier with the two T1 and T2 transistors. The quiescent current
is approximately 20 mA. The circuit will deliver a good sound
quality at an operating voltage as of 2.2V.
Summary of Contents for Do-it-yourself
Page 1: ...Do it yourself VHF Retro Radio...
Page 15: ...15 The wiring diagram The complete wiring...
Page 20: ...20...