Septmber 2005, Issue 182

Signal Generation Solution
Build an Inexpensive RF Signal Generator

CONTROLLER MODULE

The controller module is shown in Figure 5. I implemented this controller on a Microchip PICDEM 2 Plus demonstration board. The schematic shows only the portion of the demonstration board that I used. I used the prototype area to add the circuitry for controlling the RF module. The additional 8- and 20-V power supply designs are included in the schematic for completeness, but bench supplies were used for the prototype. Photo 1b shows the modified demonstration board attached to the RF module in its RF tight enclosure.

The controller includes a Microchip PIC16F877A microcontroller running at 4 MHz. The MCU’s job is light in this application, so its limited amount of horsepower didn’t matter too much.

As you can see in Figure 5, I included a two-channel MCP4922 DAC to supply the control voltages to the RF POS-2000A VCO and for the gain control to the AD8367. The DAC’s outputs were buffered using the TLE2142 low-noise op-amp. I initially attempted to use the PIC16F877A’s built-in PWMs with low-pass filtering to produce the control voltages, but I wasn’t happy with the output’s purity. It turns out that the VCO and variable gain amplifier are extremely responsive to fluctuations on these control lines. The buffered DAC with the associated MCP1541 reference source perform quite well. They provide clean, stable signals to the control lines.

The LO and RF frequency measurements are made in a unique way. The challenge is to measure a signal that’s at approximately 30 MHz with a microprocessor running at 4 MHz. This design uses the PIC16F877A’s internal Timer0 with a built-in prescaler. The prescaler is specified at 10-ns rise and fall times, which enable it to be clocked by an external source at up to 50 MHz. I gated the input to Timer0 for 1 ms and then used Timer0’s contents to calculate the frequency.

The process seems pretty straightforward, but there was a problem. Timer0 with its prescaler is 16 bits long, but the microprocessor can read only the top 8 bits. To determine the prescaler’s contents, I used the microprocessor to pulse Timer0 until the prescaler overflowed into the upper 8 bits of Timer0. The number of times you have to pulse to get to overflow enables you to identify the prescaler’s initial content. The PBasic pseudo code used to execute this is shown in Listing 1 (p. 20). A simple flowchart is posted on the FTP site.

A standard LCD and two push buttons are included. I tried to keep the interface simple, so the only controls are an Up/Down frequency button. In response to a request to change the frequency, the microprocessor will increase or decrease the control voltage to the RF VCO. The output frequencies of the RF and LO VCOs are then measured in the aforementioned fashion, and the IF output frequency is calculated. Following this, the microprocessor adjusts variable gain amplifier’s gain as a function of frequency. The microprocessor then displays the IF output frequency on the LCD.