DZ HT-7, Инструкция по сборке, Страница: 33 / 44

Page 33
The HT ‐ 7 ‐ 2 board schematic, sheet
3 (page 36), shows how we have
implemented Rob’s design. While
his design was meant for 50kHz RF
signals, today it is possible to
use very high bandwidth op ‐ amps
to implement a full ‐ wave detector
directly at the operating fre ‐
quency. The op ‐ amps chosen for
this are the Analog Devices
AD8045, with a 1GHz gain ‐
bandwidth product (GBW), and the
AD826, a dual op ‐ amp with a GBW
of 50MHz.
The AD8045 needs a high GBW be ‐
cause it is used with a gain of
11 (the 470 ohm feedback divided
by the 47 ohm input resistor,
plus 1), so at 7MHz, almost 80MHz
of GBW is needed. This gain as ‐
sures that the diodes have enough
signal to overcome their 300mV DC
voltage drop.
Both op ‐ amps are powered from a
single supply, so they are biased
at half the supply by R35 and
R36. C2 assures that the positive
input to the AD8045 is at AC
ground. R8 assures that there is
feedback around the op ‐ amp in the
region where neither D1 nor D2
are conducting, keeping it from
operating in an undefined gain
state. Its value is high so as
not to affect the gain when the
diodes are conducting.
Recall that an AM signal contains
the carrier as well as the audio
information. Demodulation pro ‐
duces a signal that must be fil ‐
tered to remove the carrier,
leaving only the audio. R10 and
C52 are used for this purpose. R7
provides a DC load for the AD826.
C50 couples the audio to the au ‐
dio amplifier consisting of U2
and surrounding parts. RV1 is the
volume control, which also has
the on/off switch integrated with
it.
Audio amplifiers require a great
deal of current to handle instan ‐
taneous voice peaks. For this
reason, a large 330uF electro ‐
lytic capacitor is placed across
the supply leads. Although the
entire transceiver draws only
about 400mA on transmit and 10mA
on receive, the supply must be
able to handle current spikes as
high as 1A! If the supply can’t
handle that much current, the au ‐
dio will sound “fuzzy” as distor ‐
tion is introduced into the out ‐
put.
C6 couples the amplified audio to
the speaker. Since the speaker
load is 8 ohms, a large value ca ‐
pacitor is required so that its
impedance is less than 1 ohm at
audio frequencies of 300 ‐ 3000Hz.
Diode D3 protects against re ‐
versed polarity.