DZ HT-7, Assembly Manual, Page: 30 / 44

Page 30
of 18 ohms, and remembering that
power is P = I
2
* R, the carrier
power is thus 0.273* 0.273 * 18 =
1.34W. The voltage that must be
developed to support this current
is given by P = E * I, or 1.34 =
E * 0.273, and thus E = 4.9Vrms,
or 14Vp ‐ p, well within the volt ‐
age available (twice the supply
or 22.2Vp ‐ p). This 14Vp ‐ p is then
transformed by T3 to 23Vp ‐ p driv ‐
ing the 50 ohm filter load, or
8.2Vrms. Ignoring the small power
loss in the transformer, the
available power is 8.2
2
/50 =
1.34W. Assuming Q3 has a gain of
about 10, a 14Vp ‐ p output re ‐
quires a 1.4Vp ‐ p input. That sig ‐
nal comes from the 2N5109 output
stage, which was, as you recall,
generating a 6Vp ‐ p signal. Trans ‐
former T2 stepped it down by a
factor of 3 to produce the needed
1.4Vp ‐ p or more. Note — because
of variations in transistor beta,
your voltages may vary from this
example by as much as 50%.
Total power in an AM signal with
100% modulation is 1.5 times the
carrier power, so a carrier power
of 1.34W translates to a total AM
power of 2W. Note that it took
about 4W of DC power to create
this (Vce * Ic = (11 ‐ .58)*0.386),
resulting in an efficiency for
this class A amplifier of 50%.
The output low pass filter is a
7 ‐ element Elliptical filter with
a cutoff frequency of 8MHz. This
frequency was chosen to get as
close to 7.29MHz as possible so
as to eliminate harmonics while
allowing for component tolerances
so that the inductors and capaci ‐
tors do not have to be adjusted.
Receiver operation
When the PTT switch, S1, is in
the receive position, supply
power is routed to the receiver
circuits and not to the transmit ‐
ter circuits (HT ‐ 7 ‐ 2 board, sheet
2 of schematics; 11.1V ‐ > RXV).
The antenna, after passing
through the same low pass filter
as the transmitter, is fed
through a high pass filter with a
cutoff frequency of 6 MHz. This
keeps local AM broadcast and many
shortwave broadcast stations from
causing interference. The result ‐
ing 6 ‐ 8MHz signal is then fed to
FIN, the same signal path taken
by the transmitter. In this case,
Q1 serves as not only an imped ‐
ance matching circuit but also as
a preamp. The output of Q1 is
then fed into the 6kHz crystal
filter which passes only the 6kHz
range of frequencies centered at
7.290MHz. The output, FOUT, is
fed via PTT switch S1 to the re ‐
ceiver circuits starting at RX
(HT ‐ 7 ‐ 2 board, sheet 3).
U1 and U4 form a full ‐ wave AM de ‐
tector. Most AM detectors are
simple half ‐ wave diode detectors.
This circuit was first proposed