The purpose of this design
project is
to provide an opportunity to
design a simple direct conversion radio receiver based on
given circuits, and to measure the performance of their design.
It consists of five modules: local oscillator, mixer, passive filter,
active filter and amplifier and audio power amplifier. Circuits are
given and a PC board, with some component values specified. There is a
degree of freedom for design in choice of components, and even some of
those that are given in the notes, if they are commonly available, can
be varied if so desired.
Oscillator
The oscillator uses a rare
5µH
slug tuned coil with a datasheet Q
of 80 but with a
much lower Q in reality. The active device is a JFET 2N5485. The
oscillator circuit is a
self-biasing Colpitts oscillator with a source bias resistor setting
the drain current and gate-source voltage (the gate is grounded at dc).
This combination makes a somewhat
marginal oscillator and careful attention needs to be paid to the
oscillation condition. The best approach might be:
- Measure the Vp and Idss of the JFET.
- Measure the coil Q, preferably using a tuned circuit at the
same
frequency of the oscillator and measuring the bandwidth.
- Find an expression for the oscillation condition using the
usual
theory of oscillators (don't blindly use any expression for a Colpitts
oscillator pulled out of a textbook - it will almost certainly be
wrong).
- Use this information to compute the source bias resistor
value at
which
oscillation will occur. An OpenOffice CALC spreadsheet model can be
built quite simply to compute the oscillation condition for a range of
bias currents from 0 to Idss. The drain resistor needs to be chosen so
that the JFET is operating in saturation mode.
- Study the oscillator behaviour: theoretically; with
simulation;
and on
a prototyping board until its behaviour is well understood. Try
different frequencies, tuned circuit combinations and bias conditions.
- Choose a value for the source bias resistor and adjust
other
components
to obtain a good oscillator output. Avoid the temptation to set this
too close to the point at which oscillations stop as it may result in
an unreliable circuit.
PSpice simulations should show
similar behaviour to the predicted
theoretical results, however in practice the oscillator may not
oscillate at the design frequency despite the insistance of PSpice.
Also the PSpice waveforms will not look anything like those obtained in
practice. The difficulties seem to be with the low inductance coil, but
the reasons for the anomolous behaviour are not clear. In
the AM band, radios normally use inductors of about 1mH and quite small
capacitances (which are in fact variable to allow tuning). These work
much more reliably with this type of circuit. Also the oscillator
output is taken from a small sniffer coil wound around the inductor.
This gives
much purer sinusoidal output although it can reduce the coil Q if the
next stage has low input impedance.
The circuit should oscillate
when the
source bias resistor is set to
zero. If it does not oscillate in this case, then you are in real
trouble.
Mixer
This should be straightforward.
The
design could be approached
by constructing a nonlinear model of the circuit in OpenOffice CALC
and adjusting the source bias resistor, and hence drain current, to
maximize
the output at the desired frequency (baseband in this case). It is a
simple nonlinear mixer (as opposed to a balanced mixer) having
significant local oscillator output that must be removed by filtering.
Note that the receiver is a
direct
conversion type, so the local
oscillator is tuned to the exact frequency of the received
transmissions.
Passive Filter
The mixer is followed by a
passive
third order low pass filter with
6kHz
cutoff and input and output impedances of 3.9K. Output impedance means
that the filter works into a load impedance of 3.9K. With equal input
and output impedances, a PI configuration may be used which just
happens to use the two 100mH inductances that are specified in the
project. If you choose an output impedance different to the input
impedance then the Butterworth filter generally has to have a T
configuration, and in any case the inductances needed will be
different. Note that the actual output impedance is the input
impedance of the following stage, and in fact things can get quite
complicated if we follow the true path. For the moment just assume it
is 3.9K.
A third-order filter is very
easy to
design if the input and output
impedances are equal. For the lazy designer, there is a particularly
easy to use filter program called FAISYN that can be used for a 30-day
trial. If you can't get your design done in 30 days, then you are not
going to make a particularly good RF designer. Make sure that you
understand the basics of filter design and the significance of
impedances. There are plenty of resources available (see Circuitsage)
including design equations and various freely available programs and
web calculators.
BJT Amplifier and Filter
The gain for this type of
amplifier
(common emitter with collector-base
feedback) is approximately given by the (negative) ratio of the
feedback impedance to the source impedance. The small signal analysis
is quite messy but in this case the approximation should hold quite
well. The source impedance is set by the previous stage (mixer and
following passive filter). This is about 3.9K in the mid-band region of
the passive filter. The value of the feedback resistor can then be
determined from the specified gain for this stage. The feedback
capacitor is easily found since the feedback RC circuit is to form a
lowpass filter with cutoff frequency 6kHz.
A problem is that the input
impedance
of the amplifier is highly
capacitive, according to Miller's theorem. This theorem states that the
feedback RC combination will appear as an equivalent RC combination
across the amplifier input, but with the impedances much reduced by the
raw amplifier gain (which is about 150 to 200 depending on biasing).
What effect
does this have on the previous passive filter? Realistically this
should be included into the design of the passive filter. However the
reality is that the output impedance of the passive filter (the input
impedance of the amplifier) is now much smaller than its input
impedance. So, if we want to have a Butterworth
filter, we would be forced to use a configuration with inductances
quite different from those provided for the project, and maybe need to
use a T configuration rather than the PI configuration required in the
project.
Looking beyond this, we see
that the
amplifier does not enhance the low
pass filter in any way. The Miller effect places the RC circuit that
determines this filter behaviour
across the input to the amplifier, where it combines with the passive
filter final capacitor. Therefore no additional pole is added. The
Miller
capacitance at the output combines with the load resistances to give a
pole at much higher frequency (about 4 times the LPF cutoff frequency)
and so has only a minor effect on the overall filter transfer function.
The recommendation is that the
feedback part of the amplifier be
omitted, and the amplifier designed to provide an approximate 4K input
impedance. The amplifier will have a small signal gain of about 160. If
lower gain is desired, an emitter degeneration resistor can be added.
This has the additional advantage that the input impedance can be more
accurately controlled.
The BJT used is not critical.
Audio Output Stage
This is basically trivial as no
design is required. It is simply a
join-the-wires exercise and is included so that an audio output can be
obtained when the radio is tuned to a station. Nevertheless it can
cause problems by oscillating at high frequency. This needs to be
investigated and controlled.
Performance
Some suggestions for
measurements:
- The oscillator output quality can be measured in terms of
the
relative
amplitudes of the fundamental and harmonics.
- The mixer output could be measured also with a fixed signal
from
a bench oscillator. Measure the mixer product and local oscillator
amplitudes.
- The filter performance can be easily measured by cutting
the
circuit after the mixer and feeding a 50 ohm audio source through a
3.9K resistor into the passive filter input.
- Using an RF bench source
the mixer and filter performance can be measured at a number of signal
levels.
A good exercise would be to
attempt
to look at third-order intercept
problems which may be significant with this architecture, but it may be
a bit advanced for this level.
Circuit Board
There are some comments to note
about
the circuit board:
- The inductor L6 is the ferrite aerial and should not be
inserted
(as with C6) until after tests have been completed.
- When L6 is connected onto the board, its common wire needs
to be
connected to ground (currently it floats in mid-air).
- The volume control pot should be wired directly under the
board,
not on top of the board.
- C5 on the circuit diagram seems to be C8 on the board.
- C13 floats in mid air and needs to be connected to a nearby
track.
- C15 would seem to be a ceramic capacitor to complement the
electrolytic C20, probably about 0.1µF.
- C14 might be the 47µF that appears some distance from
it on
the circuit diagram.
- The power to R3 should come from the 12V line, not the
mixer
filter resistor R7.
- D1, D2, L6, Z1,
TP9, C21 do not feature in the circuit diagrams and are left open.
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