The previous parts of the "How not to design transmitters and receivers" series have described all the design decisions and blocks which need to go together to produce a 1 Watt wideband FM transmitter. It is now time to start putting those blocks together into a full-blown design.

First up is the 1 Watt transmitter chain itself, composed of:

• a voltage controlled oscillator (VCO) as described in part 1
• a buffer amplifier which is not totally dissimilar to those described in part 6,
• a matching circuit between the buffer and the power amplifier, as described in part 3,
• a 1 Watt power amplifier, based on a device identified in part 2, and
• a low-pass filter, as described in part 9.

The circuit diagram below shows these elements put together (click on the image for a larger version).



The VCO should be set to oscillate at half the wanted output frequency. The '2f' component of this is then passed to the buffer which is formed of the 2SC3356. The 22pF capacitor between them is not critical. Other transistors which can produce around 100mW of power would be equally suitable in the buffering role, such as the MPSH10. The output of the buffer is fed into the power amplifier and then through a low pass filter. The output transistor (the MRF555) can also be replaced by different devices, however the matching network between the buffer and the output stage may require alteration.

No phase locked loop (PLL) circuitry is yet shown, this will follow in an ensuing post. As illustrated, the design is called a 'free running' oscillator meaning that its output frequency is set only by the control voltage applied to its input, the value of the centre-tapped inductor in the circuit, and the variable capacitor which sits across it. This design is relatively stable and after settling down has an accuracy of better than a few kHz at an output frequency of 100 MHz, as long as the control voltage remains constant and the temperature isn't allowed to drift too much.

In operation, the BF451's should draw around 10 mA each, the 2SC3356 (or equivalent) should draw around 15 mA and the output device in the region of 150mA. If constructing a prototype based on this schematic, ensure that good radio frequency (RF) construction principles are adhered to, in particular:

• The leads of all components should be kept as short as possible, and certainly not be allowed to exceed 1 cm in length. This is because a simple piece of wire will act as an inductor at RF and introduce unwanted reactance to the circuit.
• Ideally, the circuit should be built over a solid ground-plane which acts as the ground connection. A piece of un-etched copper-clad printed circuit board (PCB) material will do the job handsomely.

Note that the 10V supply which feeds both the VCO and the buffer should be regulated using, for example, one of the designs discussed in part 4.

In an earlier 'How not to design transmitters and receivers' it was blithely stated that the reference oscillator part of a phase locked loop (PLL) was a relatively trivial undertaking. And... it is. All that needs to be decided is what the reference frequency is going to be.

Throughout these series of articles, a reference frequency of 100 kHz has been used in order to illustrate how a variable frequency PLL operates. However, this would require that the output frequency is fed directly into the 'divide by N' block, and as has also been discussed, these devices do not usually operate at frequencies as high as 100 MHz, indeed many of the purpose-designed divide by N chips (such as the TC9122P) will only cope with input frequencies as high as 10 to 15 MHz. In addition, as was discussed in part 5, the lower the reference frequency, the less the PLL will see modulation as a frequency error and try and correct it. A reference frequency of 100 kHz is therefore both difficult to work with (due to the divide by N frequency limitations) and too high (due to the PLL trying to correct modulation).

How high should the reference frequency be then? In many PLL designs for FM transmitters, this is often set to be in the order of 10 kHz, though some designs are nearer 1 kHz. For the Wireless Waffle lockdown project, it was decided to use an even lower frequency, so that the low frequency audio response of the PLL could be as flat as possible.

To define the exact frequency, the amount by which the prescaler initially divides down the output frequency must be factored in. The previously discussed MB501L can be set to divide by 64 or 128. If either of these were immediately followed by a divide by N, which could count in steps of 1 and with each step being 100 kHz, then the maximum reference frequency would be either 1.5625 kHz (100 kHz / 64) or 781.25 Hz (100 kHz / 128). The reference frequency could be lower than this if, for example, the divide by N used a higher division ratio (for example, counting in steps of 2). To allow for 25 kHz steps, which are used in some other audio applications such as studio-to-transmitter links, a reference frequency of 390.625 Hz was chosen (i.e. 25 kHz / 64). Note that this means that for each 100 kHz step, the divide by N counter has to be incremented by 4, not 1.

The simplest way of generating a reference frequency is to use the tried and tested 4060 IC as shown in the circuit on the right. The 4060 has a built in oscillator (which can be used with quartz crystals such as in this application, or as just with resistors and capacitors) as well as having a binary counter that can easily produce outputs divided by multiples of 2. Even the standard CMOS version of this chip will oscillate at frequencies up to around 8 MHz with a 5 Volt power supply and higher still if the supply voltage is increased.

A crystal with a frequency of 6.4 MHz is easy to source (being a not-uncommon clock frequency) and needs to be divided by 16384 (or 2¹⁴) to produce an output at the required reference at 390.625 Hz. The 4060 has an output which divides by 2¹⁴ on pin 3 so a simple crystal oscillator and divider circuit can be made from this chip alone.

Other CMOS divider chips (such as the 4020, 4024 or 4040) can also be used for dividing down a fixed frequency oscillator but need a stand-alone crystal oscillator to drive them. An example of this arrangement is shown on the left.

Note that the value of resistors around the transistor used as the oscillator are somewhat critical. The exact current flowing through the transistor is not that critical (anywhere in the region of 2 to 5 milliAmps should be OK) however the transistor needs to be biassed such that when it is not oscillating, the collector voltage is close to half the supply voltage. This will mean that when oscillating the output swings equally above and below half the supply voltage, which the CMOS counter, whose input thresholds are similarly symmetrical, will count properly. In the case of the transistor being biassed such that the output when not oscillating sits at say, 3/4 of the supply voltage, it is possible that the peak-to-peak output swing may be reduced and may not be sufficient to drive the CMOS chip. These resistor values will be different depending on the exact transistor used (actually the specific DC current gain, β, or h_FE of the transistor). As with many electronic design problems these days, there's an online tool to help calculate the necessary resistor values..

And so, the end is near, and that final curtain is nearly ready to be opened to a blaze of audience applause. In the next article in this series, everything learnt so far will be soldered together into a single circuit diagram (or schematic, whichever is your preference). And then it will be time to start to think about receivers, and their associated building blocks. Some say that radio frequency (RF) design is a bit of a black art, insofar as there are so many hurdles which can trip you up along the way (if that isn't a badly mixed metaphor). For transmitters, the biggest problems come from unwanted harmonics and crazy spurious oscillations. In receivers there are these things to deal with, but also the need to have a nice clean and distortion path from the input to the output so that the wanted signal can be received, and any unwanted signals can be rejected. Nobody said it was easy.

Before heading down the final straight of the Wireless Waffle lockdown transmitter project, and mounting the horse of the associated receiver, a few words on varicap diodes, or varactors as they're also known.

A varicap is a diode which, when reverse biassed (i.e. not conducting) acts as a capacitor. Varying the voltage of the reverse bias, affects the amount of capacitance that the device exhibits. The higher the reverse voltage, the smaller the capacitance. These, are therefore the devices which enable an oscillator's frequency to be tuned with a voltage. Replacing the capacitor in a tuned circuit with a varicap allows the resonant frequency (and thus the frequency of oscillation) to be varied, making a voltage controlled oscillator (VCO).

However, not all varicaps are made the same. The amount of capacitance they exhibit can be very different. Some are designed for use in medium-wave circuits and can have several 100pF of capacitance. Some are designed for UHF and have only a few picoFarads (pF) of capacitance. Some operate only over a relatively narrow range of voltages, some require a very wide range of voltages to perform to their best. There are also differences in the extent to which the capacitance changes between a low reverse voltage and a high one. Some varicaps will have 10 or more times less capacitance at a high voltage compared (say 28 Volts) to a low one (say 1 Volt), whereas others may only have a high to low capacitance ratio of nearer 4.

For use in a VHF VCO being discussed in this series, a varicap with a capacitance varying from around 20pF to 10pF would be about right. Paired with an inductor of around 120nH and together with the stray capacitances of the oscillator circuit, this will enable oscillation over the frequency range of interest. To resonate a 120nH coil at 87.5 MHz requires a parallel capacitance of 27.5pF, falling to 18pF at 108 MHz. So a varicap which can tune from 20pF to 10pF in parallel with around 8pF of stray capacitance would be spot on.



There are a wide range of varicap diodes which can perform this job, and an equal number which can't. Trying to find data online for some devices can prove very difficult as many which are cheaply available are of a vintage where few datasheets remain, and some are originally of Japanese manufacture meaning that the datasheets are primarily in Japanese. Below is as many devices as have been found which ought to be able to do the job in hand. Note that all of these devices are in a two lead, glass-body package (such as the good old 1N4148). There are surface mount devices (SMD) available, and some odd square packages but these have not been listed, due to their lack of aesthetic pleasure-giving.

Device	Specified Use	Maximum Capacitance and Voltage	Minimum Capacitance and Voltage
BB139	VHF Tuner	29pF@3V	6pF@30V
BB329	VHF & CATV Tuner	35pF@1V	3pF@28V
BB409	VHF Tuner	27pF@3V	6pF@25V
BB609	VHF & CATV Tuners	33pf@1V	3pf@28V
BB809	VHF Tuners (up to 160 MHz)	42pF@1V	4.5pf@28V
BB909	VHF Tuners (up to 460 MHz)	24pF@1V	3pf@28V
1N3182	VHF Tuning	33pf@4V	?
1S2688	VHF AFV	20pF@1V	2.2pf@10V
1SV68	VHF/FM Tuner	32pf@3V	5pF@25V
1SV97	VHF Tuning	25pf@?V	?
1SV113	VHF Tuning	21pF@3V	3pf@25V
1SV124	VHF Tuning	35pF@1V	3pF@28V
1SV126	FM/VHF Tuning	unknown
1SV155	VHF Tuning	unknown
1SV184	FM/VHF Tuning	50pf@2V	4-6pf@10V

Incidentally, if anyone knows of any other devices which can do the job, please post them in the comments and they can be added to the table. Also, if anyone has the precise details of any of the diodes whose parameters are 'unknown', any information on them would be greatly received.

In the VCO design presented in part 1 of this series, two varicaps are used in a back-to-back arrangement. It is important that these two are relatively well matched. Some of the older diodes listed above (such as the BB139 or 1N3182) are not well matched, and the datasheet shows that the exact capacitance value provided at a particular voltage can be as much as 30% different between two devices. This either means measuring and selecting two similar diodes or choosing more modern components whose tolerances are much better. Of course more modern devices are often more expensive, so like many things, there's a trade-off to be had.

Many designs for transmitters and power amplifiers which can be found online (certainly some of the older designs such as the one below) fail to include any low pass filtering on their outputs.



There are several possible reasons for this:

• maybe it was expected that the avid constructor would know to add their own filter;
• maybe it was thought that any antenna connected to the device would magically filter out the harmonics (actually, this can sometimes happen for even multiple harmonics such as the 2nd, 4th and 6th, but is not the case for the odd ones);
• maybe (for older designs) there was such light use of radio frequencies that it didn't matter if your transmitter also radiated on a bunch of other frequencies.

Let's first look at what services occupy the 2nd to 5th harmonic frequencies of the FM band (i.e. multiples of 87.5 to 108 MHz).

Harmonic	Frequencies	Services
2nd	175 - 216 MHz	DAB Radio, PMR
3rd	262.5 - 324 MHz	Military Aircraft
4th	350 - 432 MHz	Military Aircraft, Blue Light Communications, Emergency Distress Beacons, PMR, Radio Amateurs
5th	437.5 - 540 MHz	Radio Amateurs, PMR, Television

Of course the specific frequency of operation will determine exactly which service may be affected. It's not as if a transmitter produces harmonics on all these frequencies simultaneously. However there are some bad options within this range. Imagine a transmitter on the roof of a building where there is a DAB receiving antenna, the 2nd harmonic of the transmitter could wipe out DAB reception, causing annoyance to those trying to listen to the extremely poor quality, low bit-rate audio services that DAB provides. Even worse is the 3rd harmonic. There is no way in which you want to be tracked down by the military for interfering with their aeronautical communications and, as previously mentioned, something like a simple dipole will radiate perfectly well at the 3rd harmonic as well as the fundamental frequency.

The solution is to fit a filter at the output of the transmitter. The purpose of this filter is simply to reduce the amount of harmonics which get fed to the antenna. One simple solution to getting rid of even order harmonics was previously presented here at Wireless Waffle, however this is a narrow-band frequency specific design and although it may provide very good rejection at a particular frequency, it would not be as good when considering harmonics across a range of possible frequencies.

This is where a straightforward low-pass filter comes into its own. At radio frequencies (RF) such a filter is made up of capacitors and inductors. Each element of the circuit increases it's 'order'. So a circuit with two capacitors and one inductor is known as a third-order filter. The more elements a filter is made up of, the better it will perform at getting rid of harmonics. Great care, however, has to be taken not to also impact upon the frequencies you wish to allow through the filter (for example 87.5 to 108 MHz) as, if badly constructed, filters can also reduce the level of these wanted signals.

Low-pass filter design is an art form in itself. There are many different types of filter all with different characteristics.

• A 'Bessel' filter is designed to have the flattest possible phase response, however it is not very good at reducing harmonics. Bessel filters are almost never used for RF filters.
• A 'Butterworth' filter is designed to have the flattest possible frequency response across the wanted frequency range. It is better at filtering harmonics than the Bessel filter, but is still not that great compared to some other options.
• A 'Chebyshev' filter is designed to allow a trade-off between the flatness of the response across the wanted frequency range, and the amount of harmonic filtering. These filters introduce ripple in the wanted pass-band but have much better harmonic rejection.
• A 'Cauer' or 'Elliptic' filter not only introduces ripple in the wanted frequency range, but also in the rejected frequency range. The harmonic rejection of these filters can be tuned to be excellent at specific frequencies.

A good example of the relative performance of these filter types is presented in Circuit Note 0304 from Analog Devices. One added complication when designing filters is that the better its performance at rejecting harmonics, the more finicky it will be to build and replicate. There is thus a limit to what can be achieved if a design is to be repeatable, can use capacitors of a manageable tolerance (i.e. 5% or 10%) and does not need to be hand-tuned. Some Cauer filters are particularly difficult in this regard and would need a network analyser to correctly set them up. Another design difficulty is that if you do not wish to have any variable elements (i.e. variable capacitors) then the design of the filter needs to be able to use standard capacitor values (i.e. the E12 series), or at least be able to have the required capacitor values made up of say 2, E12 values in parallel (i.e. 15pF and 15pF to make 30pF). If inductors are being hand wound, then their value can be more easily tailored through changing the number of turns, thickness of wire, winding diameter and turns spacing.

Thankfully there is amazingly useful filter design tool available online (the RF Tools LC Filter Design Tool to be precise). For the Wireless Waffle lockdown project, a 7 pole Chebyshev filter with a pass-band ripple of 0.06dB and a cut-off frequency of 113 MHz was selected. This balances performance against complexity and simplicity as, in particular, the capacitor values required are from the standard E12 series (if the 30 pF capacitors are made of two 15 pF capacitors in parallel). The design is presented below.



When assessing filter performance for an FM transmitter, given that the frequency reponse drops linearly (except for Cauer filters) as frequency increases, the worst performance will be at harmonics of 87.5 MHz. Whatever is achieved at that frequency will be bettered at harmonics of 108 MHz which is higher in frequency. The filter illustrated above has a largely flat frequency response to around 113 MHz, and then attenuates harmonics as follows.

Harmonic	Frequencies	Attenuation
2nd	175 MHz 216 MHz	36 dB 51 dB
3rd	262.5 MHz 324 MHz	64 dB 78 dB
4th	350 MHz 432 MHz	Better than 80 dB

Note that these ideal, calculated values will be limited by the method of construction. A filter built on printed circuit board (PCB) in limited space will never perform as well as one constructed in a seperate metal can with each inductor isolated from its neighbours. In practice, a compact, PCB based low-pass filter would struggle to achieve much more than around 60 to 70 dB harmonic rejection but for low power transmitters, this should be sufficient (and indeed meets the Ofcom code of practice) which for a 1 Watt transmitter requires that harmonics and spurious should be 46 dB below the carrier. Given that the second harmonic of the raw FM transmitter, even at 175 MHz, is already more than 10 dB below the carrier the additional 36 dB of attenuation provided by the filter will meet the Ofcom requirements with no difficulties.