The 'Wireless Revolution' is sure making it convenient for amateurs who like to make radios! Every week, a fistful of new, highly integrated receiver chips is introduced. With today's 'Experimenter Friendly' distribution channels it is even relatively easy to get parts, in fact many manufacturers have Internet web sites where samples can be ordered directly. Perhaps the only downside to this revolution is that the size of the parts keeps shrinking.

This receiver is an example of using a commercial chip designed for cordless phones to build a highly integrated 2 meter FM receiver. The receiver can be operated as a portable scanner or connected to a PC's RS232 port and be operated under PC control.

RF Deck

The heart of any radio is the RF deck, everything else is support circuitry, and this receiver is no exception. Many projects in the past have used the Motorola [1] MC336X type device. This part was recently superseded with the more highly integrated MC13135 family. Truly a 'receiver on a chip' the MC13135 contains almost all the functions to get RF energy converted to audio and even has relatively good large signal handling capability.

As can be seen in the receiver schematic, the support circuitry required for the MC13135 is minimal. The design uses the MC13135 in a dual conversion configuration with an external 1st LO and internal, crystal controlled 2nd LO.

The 50 ohm, RF from the antenna is matched to the 700 ohm input impedance of the MC13135 by a double tuned LC filter. This filter is fixed tuned to cover the entire 2 meter band. C1, C2 and L1 step the 50 ohm antenna impedance up to around 700 ohms to match the MC13135's input impedance. The second section, C5 and L2 add more selectivity. C3 and C4 were chosen to set the degree of coupling and hence the bandwidth of the filter. I used two capacitors in series here to allow for more adjustability in setting the bandwidth while using standard leaded capacitor values.

The first mixer in the MC13135 is a classic active Gilbert cell. The main drawback of these active mixers is their low third order intercept. This limits the large signal handling performance of the receiver. On the plus side however the active mixer provides conversion gain so no amplifier needs to be added after it and the sensitivity of the mixer to output match problems is reduced over a double balanced diode configuration. Gilbert cells also operate with a very low power LO input, this makes the layout much less sensitive to spurious signal's being picked up due to the low 1st LO power being used.

A tradeoff was made in the choice of LO frequencies. The 1st IF frequency was chosen to be 10.7 MHz. This frequency was picked to make the receiver easy to acquire parts for. A 21.4 MHz first IF would have made the image rejection better, but crystal filters for these IF's are not that easy to come by. With the first IF at 10.7 MHz, the image frequency is around 165-169 MHz. These image frequencies don't present much of a problem as the 165-169 MHz band is between the common 'dreaded' pager frequencies.

The MC13135 has an internal transistor that can be used for the first LO, but it's operation is limited above 100 MHz, so a discrete transistor (Q1) was used instead. Q1 is configured as a Colpitts oscillator with C20 and C21 setting the amount of feedback. C22 provides a DC block to L4. C23 is used in series with the tuning diode D1 to limit the tuning range that D1 would provide on it's own. When building a VCO it is desirable for noise reasons to have the tuning range be as small as possible. Since the usable, linear tuning signal range from the PLL is a fixed 1 to 4 volts (set by the power supply voltage) the VCO is adjusted to just cover the LO frequency range required. Decreasing the tuning sensitivity of the VCO improves the noise performance of the VCO due to noise on the VCO tuning voltage line.

The 1st LO VCO is tuned to the input frequency plus 10.7 MHz for high side injection to the mixer. This produces a constant 1st IF frequency of 10.7 Mhz. The output of the 1st Mixer (Pin 19) is impedance matched to the 10.7 MHz, two pole crystal by resistors R2 and R3. The two pole crystal filter specified has a 3 dB bandwidth of +/-4 kHz and a stopband of 20 dB at +/- 18 kHz.

The second mixer operates with a 10.245 MHz LO frequency to convert the 10.7 Mhz first IF to 455 kHz. The second IF filter is a low cost 12 kHz bandwidth unit that provides more selectivity and limits the noise bandwidth to the input of the limiting amplifiers. A narrower filter could be used here, but the 12 kHz unit allows for frequency drift in the 2nd LO and provides more than adequate stopband performance. The MC13135 is so well thought out that it's impedances match those required by the 2nd IF ceramic filter without any external components. At the output of the second IF filter, the total gain of the MC13135, minus the filter losses is about 12 dB. The total Noise Figure of the receiver at this point is about 18 dB.

This level of performance is sufficient for 3-5 microvolt sensitivity. The only real drawback of these integrated receivers is the relatively low third order intercept of the first mixer of -17 dBm. This manifests itself as a limiting factor in the receivers overall Spurious Free Dynamic Range.

One way to define Spurious Free Dynamic range (SFDR) is the ratio of the Minimum Detectable Signal (MDS) to the signal level that produces third order products that are equal to the MDS. In this receiver the spurious free dynamic range is about 60 dB. That may sound small, but in absolute voltages the ratio is 1000:1. So for a 1 uV MDS, the largest spurious free signal that can be processed is 1 mV (all in RMS volts). Larger signals than the theoretical SFDR predicts can be present at the input and a readable signal can be heard, but the distortion products will decrease the signal to noise ratio.

After the second mixer IF filter the MC13135 has 110 dB of gain in the limiting amplifiers. This much gain usually presents an excellent opportunity to cause regenerative feedback and oscillation in the amplifiers. The MC13135 prevents some of these problems by rolling off the bandwidth of the limiting amplifier above 2 MHz, use of a ground plane when building also helps to keep the MC13135 oscillation free.

The limiting amplifier produces a logarithmic received signal strength output (Received signal strength indicator or RSSI) that produces a voltage that is proportional to the logarithm of the input RF signal. The RSSI signal is buffered by an uncommitted amplifier in the MC13135 (pins 14, 15 and 16) and fed to one of the A/D channels in the PIC uP. This RSSI signal is then used for squelch and providing the receiver with a signal strength indicator on the LCD display.

The FM signal is demodulated to audio by a typical integrated quadrature demodulator contained in the MC13135. The 90 degree phase shift required for the demodulation is provided by L3, a commercial quadrature coil.

PLL Circuit

IC2 is a Motorola MC145170, fully programmable Phase Locked Loop that is controlled by the PIC uP. The PLL is what gives the receiver the ability to be 'Digital' that is the LCD display can display the exact frequency that the receiver is tuned to without actually counting the LO frequency. This is done in the PLL by dividing down the 10 MHz crystal reference clock to 5 kHz with the 'R' counter (divide by 2000). This 5 kHz reference frequency then sets the minimum receiver channel spacing. From 144 to 148 MHz there are 801 such channel steps. The uP keeps track of what channel the receiver should be tuned to and programs the main PLL counter ('N' counter) accordingly. When the N counter is varied from 30,940 to 31,740 the 1st LO tunes from 154.7 to 158.7 MHz.

The PLL also has a buffered output that is routed over to the PIC uP for use as it's clock (through the 'C' counter). Since the PIC is doing RS232 communications it must have a stable clock source also, but the required frequency accuracy that the PIC requires for successful RS232 is about 60 times less than the RF portion needs.

The PLL feedback loop is stabilized by the loop filter composed of components R12, R13, R14 and C28, C29 and C24. R12, R13, R14 and C29 add the main pole and zero to the PLL transfer function. These are the most important components in stabilizing the loop. C28 and C24 help to filter any reference feedthrough from modulating the VCO. The effect of C28 and C24 is to actually destabilize the loop by adding more poles to the overall transfer function. These components were taken into account during the design of the loop to ensure stability.

PIC Microprocessor

The user interface is managed by a Microchip Technology [2] PIC 16C73 microprocessor. The PIC provides the means to have a very comprehensive set of features for the radio with minimum parts count. This PIC contains two 8 bit I/O ports, an internal 4 channel, 8 bit A/D converter and RS232 UART, all in a 28 pin package. For a detailed look into what it takes to develop projects using a PIC see the box "PIC Development on a Shoestring".

The radio operates in two basic modes: The first is standalone mode. When the radio is first turned on it checks to see if the RS232 port is connected. If it is not connected the radio goes into standalone mode. In this mode the radio operates like a portable receiver and all the knobs, switches, and LCD are active.

If the PIC senses that the RS232 is connected on powerup it enters RS232 mode. In this mode the PIC ignores the knobs and buttons and waits for RS232 commands to control the receiver functions.

In standalone mode, the PIC continually loops looking for any input in the tuning knob encoder, switches, RSSI value and squelch setting. If it senses any change in these inputs it takes the appropriate action. The receiver provides these functions,

 

* Volume level (not PIC controlled)

* Squelch level -- The PIC compares the level set here with the RSSI value from the receiver

and determines whether the audio should be on or off.

* Tuning Step Size in 5, 10, 100 kHz and 1 MHz steps along with a lock setting which locks the

frequency to the current setting.

* Scan button, which places the scanner in a scan mode in which it steps through the entire band

and delays on any active signal.

* Tune encoder which allows an 'Analog' frequency tuning input via a front panel knob.

The LCD displays the currently tuned frequency in MHz and the current tuning step size selected. In addition the second line of the LCD acts as a bargraph signal strength meter.

PC Control Program [5]

A PC control program has been written which allows total control of the receiver from a PC. The PIC responds to RS232 commands by setting receiver hardware as needed and sending back receiver status information to the PC. The PC program has three major modes,

First is the manual / scan screen, in this mode the receiver may be tuned to any frequency in the two meter band. Also in this mode a straight band scan may be performed. Another mode allowed by the PC program is a memory scan, in this mode the operator loads up to 10 frequencies into the computers memory and the PC scans only these set frequencies.

Perhaps the most interesting mode is the spectrum analyzer mode, in spectrum analyzer mode the PC continually sweeps the 2 meter band in 10 kHz steps and displays the RSSI value at each point. This generates a relative signal strength plot versus frequency. A cursor allows actual frequencies to be determined and by double clicking on a peak that frequency is tuned to.

Usually when one composes a program, all sorts of 'constants' get hard coded into the program. The constants contain display scaling factors and things like the delay to wait after a signal stops before scanning resumes. These decisions can make the radio inconvenient for other peoples preferences. I added an 'options' screen to this program that let's the user override all my preconceived constants with their own. Every time the program starts these preferences are reloaded from disk.

 

Building/Options

The receiver can be built as shown or it can be built just for RS232 operation with the PC control program. If PC operation only is desired several parts may be omitted,

1) The LCD display may be left off as the LCD is not used in RS232 mode.

2) The tuning encoder, scan and step switches are not used.

3) The squelch pot may be omitted.

The radio then works only under PC control and the only adjustment left on the radio is the volume control.

Building the radio is straight forward on the FAR [3] printed circuit boards. The PCB was designed for through hole versions of IC1 and IC2, if these are unavailable the alternative is to get surface mount types and use the Aries SMT to DIP adapter sockets that are available (see parts list). This provides maximum flexibility in the construction of the receiver.

Several jumpers are used on the PCB, see the documentation file for the correct placement.

Special care must be used on one jumper in particular, the jumper from R16 at IC2 to IC4 is the 10 MHz clock line for the PIC. This jumper has an associated ground trace on the PCB that it is designed to be routed on. By routing this jumper on the ground plane a kind of Poor Man's coaxial cable is made, this helps to keep the clock signal clean and prevents 10 MHz harmonic radiation to the RF deck. The jumper can be made from a small length of wire wrap wire. Tack the wire along the ground trace every 1/2 inch or so with super glue or some similar adhesive.

The inductors L1, L2 and L4 are Toko Surface mount types. They are positioned over the PCB and one end is soldered to the ground plane, the other end is then soldered to the pad on the top of the PCB.

Enclosure

Of course you can house the project in any suitable enclosure, but the project has been designed to fit into a professional looking Pac-Tec [4], hand held enclosure. All the pots, speaker, battery, display and RS232 connector can be 'shoe horned' into the case without too much trouble.

There is just enough room in the Pac-Tec enclosure to fit 5 Nickel Metal Hydride AAA cells. These batteries (see parts list) have a rating of 550 mA-Hours and can power the receiver for 5 - 10 hours depending mainly on the audio setting used. The 5 cells have a nominal voltage of 6 volts, they may be recharged in the receiver by any convenient DC wall wart type of supply. I recycled a 12 volt, 200 mA DC wall wart and used a 36 ohm, 2 watt resistor to limit the charging current.

On my portable receiver I used a 5 pin DIN receptacle mounted in the enclosure to supply both charging power to the batteries and make the RS232 connection when needed. I mounted the charging resistor in the mating DIN plug.

 

Tuneup

The receiver is relatively simple to tune up. First set the PLL voltage by setting the receive frequency to 148.000 MHz. Adjust L4 until the voltage at R14 is about 3 volts. Now set the receive frequency to 144.000 MHz, the voltage at R14 should be around 1.5 volts.

The 10 MHz reference is set by tuning the receiver to a known station frequency (like the local clubs repeater frequency), then adjusting C32 for the best signal.

The FM demodulator should be set fairly close to ideal from the factory. You can optimize the audio quality by listening to a good signal and adjusting L3 slightly. Do not turn L3 more than 1/2 a turn either way or you may actually get on the "other side" of the demodulation 'S' curve which will actually reduce the audio level and quality.

The input filter is probably the hardest adjustment. Of course if you have access to a network analyzer use it. But I'm betting most people won't, so here is the 'manual' procedure. It will be best if you pretune the filter first. Set L1 so that the tuning slug is 3/4 of the way to the bottom of the core. Set L2 so that 1/4 of the slug is above the top of the core. Now turn to a station that is around mid-band. Adjust L1 for a signal peak. Now adjust L2 for the best signal levels at 144 and 148 MHz. You may need to adjust L1 slightly to optimize the bandwidth, then readjust L2. L1 basically sets the center frequency of the filter and L2 adjusts the bandwidth.

The signal source can be a signal generator, a dip meter (with a frequency counter) or another transmitter at some distance from the receiver. To prevent mis-adjustment due to overloading, make sure that the LCD signal meter is at 1/2 scale or less during L1 and L2 tuning.

References:

[1] Motorola Inc, Phoenix Arizona, www.motorola.com (now ON Semiconductor, www.onsemi.com)

[2] Microchip Technology, Chandler Arizona, www.microchip.com

[3] FAR Circuits, Dundee, IL, www.cl.ais.net/farcir

[4] Pac Tec, Concordville, PA, www.pactecenclosures.com

[5] Program may be obtained free from the ARRL FTP site see the FAQ page.

All commercial rights reserved.

Originally published in QST, February 1999 and copyrighted by the ARRL/QST.