Stable FM microphone
FM exciter using standard 4000 series CMOS logic
Introduction
Simple LC stabilized transmitters are invariably plagued by both frequency drift & hand proximity effect.
In the mid 90s, we investigated stabilization using the common 10.7 MHz ceramic filters. The ceramic filter's modest Q (Q = fc / BW = 10.7 / 0.180 = 59) offers enough pullability for wideband FM (WBFM). In contrast, a 10.7 MHz crystal has much higher Q and so, cannot be pulled sufficiently for WBFM.
Material & method
The architecture of the filter-stabilized transmitter was inspired by WB0NQM's 7 MHz transceiver built around 74LS00 AND gates [1]. However, instead of the 74LS00, we opted for the CD4049 hex invertor. This choice was made because standard logic ICs contained 6 inverters versus 4 AND gates. So, in just one logic IC, there are 6 invertors that could be repurposed into audio amps, active LPF, oscillator and frequency multiplier (fig. 1).
The CD4069 is another hex inverter, but is unbuffered. We chose the 4049 buffered inverter instead of the 4069 inverter because we suspect the former's larger output current can allow the oscillator & frequency multiplier stages to generate stronger signals.
Fig. 1: Stabilized FM Tx based on CD4049 hex inverters for audio amplification, active LPF, oscillator and frequency multiplier
The condensor microphone's output is amplified by a pair of inverters, Q1a & Q1b. To enable the inverters to function as audio amplifiers, they were linearized with negative feedback via R3 & R4. Using 33kR feedback resistances, the cascaded gain is ~100 at 1 kHz (fig. 2).
The amplifier output is connected to a pair of diodes. The diodes serve to clip the peaks of the audio signal, thereby limiting the maximum FM deviation.
The third inverter Q1c is configured as a low pass filter (LPF) with a 3.2 kHz cutoff. The LPF reduces the harmonics generated by the audio clipping diodes D1 & D2. As the LPF has a gain of ~5, the total audio gain is ~ 500. Due to the limited baseband frequency range, we dispenses with preemphasis for simplicity.
Fig. 2: Gain provided by the audio amp and low pass filter stages
The filtered audio signal then modulates the varicap pair D3. The varicap's quiescent bias voltage (~5.6V) is provided by a zener diode D4. The zener diode ensures that the varicap quiescent voltage, hence, the frequency, does not change with supply voltage fluctuation; i.e. frequency pushing.
The oscillator Q1d is a Pierce using an inverter. The inverter is biased into the linear region by R9. The ceramic filter is "pulled" by a pair of varicap diode D3. This varicap is commonly used for FM radio tuning.
An alternate connection for the varicap diodes is shown in fig. 3. This places the varicap diode in the filter's earth path. The resultant frequency is slightly higher than the first.
An alternate connection for the varicap diodes is shown in fig. 3. This places the varicap diode in the filter's earth path. The resultant frequency is slightly higher than the first.
Fig. 3: alternate connection for the varicap diode
Ordinarily, higher-order harmonic multipliers, such as 9x, are very inefficient and so, good design would call for a cascade of 3x multipliers instead. However, an exception was made here in order to minimize the number of resonators & stages. Two cascaded inverters drive the opposite ends of the output tank L1 in order to replicate a push-pull configuration. Push-pull was chosen because it preferentially produces odd-order harmonics.
The multiplier's output is amplified by a BF987 Si MOSFET (Q2) which was the standard RF preamp used in 90s era Bosch / Blaupunkt radios.
The module was assembled on paper phenolic FR2 single sided PCB (fig. 3) drawn with DOS-based EasyPC layout software.
Fig. 4: The prototype assembled on FR2 single-sided PCB measuring 5 x 5 cm. The ceramic filter F1 is mounted on sockets so that it can be swapped to change the frequency.
Fig. 5: Part placement diagram
Table 1: part list
The ceramic filter was mounted on the PCB using sockets so that they could be swapped to change the frequency. Murata ceramic filters have colour dots to indicate their centre frequencies. Using an assortment of red, orange and blue dotted filters, the frequency could be changed over 97.1~97.6 MHz (fig. 6).
Fig. 6: Ceramic filter colour marking. Table of an assortment of red, orange and blue dotted filters and their resultant frequencies
Results
The prototype demonstrated both frequency stability & freedom from proximity effects. From power up to 1 hour later, the frequency drifted a maximum of 11 kHz (fig. 7 & 8). This amount of drift is unnoticeable compared to the FM IF channel bandwidth of 180 kHz. The drift probably originates from the varicap rather than the ceramic filter. Aside from the target application as transmitter, the frequency stability also allows it to be used for testing broadcast FM receivers.
Fig. 7: Good frequency stability from power up to 1 hour later.
Fig. 8: From power up to 1 hour later, the frequency drifted a maximum of 11 kHz - an unnoticeable amount in the 180 kHz channel bandwidth
The prototype has enough power to cover a modest size home and also spectral purity to meet regulations. The output power is -5 dBm (fig. 9). To comply with local regulations, an attenuator can be connected to the output. Other harmonics are suppressed by >50 dB wrt the desired 9th harmonic.
Fig. 9: Output spectrum of the prototype showing a 5 dBm signal and other harmonics suppressed by > 50 dBc
A 2mVrms (~5.6 mVpp) signal at the microphone output will result in a ~3V change at the varicap voltage. The corresponding FM deviation is +/- 20 kHz (fig. 10).
It may be possible to modify the design for ham 6m band by operating the multiplier on the 5th harmonic.
Fig. 10: Frequency versus varicap voltage Vt
References
[1] R. Lucas, "TTL transceiver for 40 meters", 73, Nov. 1990.