Dual-band transceiver for 1.3GHz and 2.3GHz bands

btty038

Dual-band transceiver for 1.3GHz and 2.3GHz bands [Copy link]

This is a low power dual band transceiver designed for the 1.3GHz and 2.3GHz amateur bands, operating in both SSB and CW modes. The frequencies covered are 1296MHz to 1298MHz and 2320MHz to 2322MHz.

Schematic diagram

On the receiving end, the signal is amplified by two low noise amplifiers (Q1 and Q11), one for each band. These are made using BFP650 silicon germanium transistors . The gain of the LNA is about 14dB and the noise figure is about 1dB. After the LNA a microstrip bandpass filter is added, which filters out the unwanted signals. The design of these filters requires a good understanding of the PCB material properties (Er, thickness, etc.). Even small changes in these parameters can change the center frequency of the filter and thus degrade the performance of the transceiver. All filters use a switching diode (DAN235 or any other equivalent type) to switch between RX and TX mode.

The first mixer is a Minicircuits RAY-11 . This is the most expensive component in the transceiver, but can be replaced with any other diode mixer that covers these frequencies.

The frequency of the first IF is 272MHz. The reason for choosing this frequency is to simplify the design of the first LO. The 272MHz IF amplifier uses BFP650 and MOSFET BF998, controlled by AGC.

The first LO frequency for the 1.3GHz band is 1024MHz and for the 2.3GHz band is 2048MHz, twice as high as 1024MHz. Starting with the same crystal oscillator and using cascaded multipliers allows the first mixer to be fed with the same LO but picked up from different stages.

The RAY-11 mixer and the first IF bandpass filter are switched between RX and TX modes using diodes (D3, D4).

Q2 and Q3 are the first IF amplifier and the second RX mixer is a dual-gate MOS transistor (Q4).

The second IF is 10.7MHz and the main channel filtering is done using a crystal filter (BW = 3kHz). This filter is switched between RX and TX using diodes (D5 and D6). In SSB-TX mode, this filter is used to suppress the unwanted sidebands and residual carrier remaining after DSB modulation.

Q5 and Q6 are the second IF amplifiers, which are controlled by the AGC.

U1 ( SA612 ) has triple functionality. It is a product detector in RX mode, a DSB modulator in TX mode and a beat frequency oscillator using an external 10.7MHz crystal. Adjustment of the C25 BFO frequency can be set in the right position on the Crystal Filter side. Q8 also has dual functionality, an audio preamplifier in RX mode and a DSB buffer in TX mode.

In TX mode, the 10.7MHz SSB signal is filtered by the crystal filter and amplified by U10. The signal is then converted to 272MHz using mixer U11 (SA612).

The second LO used to convert between the first and second IF is a PLL or a DDS covering frequencies from 261.3MHz to 263.3MHz. The schematic for this LO is not shown here. I myself used a HF DDS (AD9850) to upconvert to the required frequency.

block diagram

Main Schematic

Microstrip Filter