|
One of the double balanced mixer and oscillator
ICs which is commonly found in HF-VHF receiver designs is the NE/SA602 or
612. These chips contain a gilbert cell mixer and provide about 14 dB of gain below
45MHz. This part will be suitable for
both AM and SSB demodulation of my 9MHz IF.
The main concern with this design is to limit the RF input level to
guarantee a low level of 3rd order products. From the datasheet, somewhere between
-60dBm and -40dBm seems reasonable.
This will give me an output signal level of 200mV for -60dBm input and
2V for a -40dBm input. Input Voltage from dBm level dBm = 10 * log(Pmw) = 10*log( V^2 /
1.5kΩ)
for -60dBm, V = 40mV for -40dBm, V = 400mV Synchronous Demodulation
(AM) When listening to AM signals, it is known that the carrier and two sidebands fade out of phase with each other. The synchronous demodulator improves the comfort level of listening to AM signals over those demodulated with a basic envelope detector. In synchronous demodulation, the carrier is reinserted and used to demodulate the original carrier signal. The circuit will phase lock to the original carrier signal. Product Detector (SSB) The product detector operates at the IF
frequency to demodulate the signal into audio. A crystal is used to generate the oscillator
mixing frequency. The crystal is needs
to be switched when going between USB and LSB modes. |
|
|