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QRZ! Ham Radio 17
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From : KC6HPN @ WB6YMH.#SOCA.CA.USA.NA
To : 9600 @ ALLUS
Date : 920426/2249
Subject: TAPR 9600 MODEM MODS PART 1
Path : K6VE!WB6YMH
From: KC6HPN @ WB6YMH.#SOCA.CA
TAPR FULL-DUPLEX (NEW) 9600 BAUD MODEM MODS BY KC6HPN (PART 1 OF 4)
APRIL 23, 1992
These modifications correct several problems that can seriously degrade
the performance of the new (not the K9NG) TAPR 9600 baud modem. They have
significantly improved the usability of my modem. DCD (Data Carrier Detect)
operation, in particular, went from intermittent to rock solid. All mods have
been implemented on my (first production run) TAPR modem board, which is
installed in a PK232MBX.
I welcome any comments and will answer any questions on these
modifications. Just drop a message in my BBS mailbox,
KC6HPN @ WB6YMH.#SOCAL.CA.USA.NA.
73, Brian, KC6HPN
MODULATOR SIDE MODIFICATION:
1. HOW: Remove capacitor C5 (0.001 uf).
WHY: Op-amps tend to oscillate when driving a non-isolated capacitive
load to ground. U22B, a TLC274 op-amp is driving C5 to ground through C31.
U22B may oscillate at approximately 400 kHz due to the pole created by C5.
This oscillation will be superimposed on the modulating waveform. To diagnose
this problem, examine the eye pattern at TXA. The eye should be sharp, if it
is fuzzy, the op-amp is oscillating.
NOTE: C5 normally functions as part of an RFI filter to prevent noise
from entering the modem via the TXA line. I have found that in my situation
C2 and R2 alone are sufficient. Those with RFI problems should not remove C5
Instead, place a 220 to 470 ohm resistor in series with C31 to provide
resistive isolation from C5. This resistor will cause a slight loss in output
drive. If necessary, you may increase output drive by changing R39 to 300 k.
NOTE: The first TAPR kits used a TL084 op-amp for U22 which may be
slightly more resistant to oscillation. However, the TL084 may still
oscillate.
DEMODULATOR SIDE MODIFICATION:
1. HOW: Change U4 from a TL084 to a TLC274 op-amp.
Replace D2 (1N4148) with a germanium diode (Bvr > 15 volts)
or if not available with a 1N3600 silicon diode.
WHY: Noise margin. The data slicer output is extremely dirty. An
increase in noise margin represents improved bit recovery performance.
The TLC274 can pull its output nearly to ground, the TL084 cannot. This
becomes important in U4A, where the op-amp is used as a schmitt-trigger
comparator (data slicer). The op-amp output drives a digital input to ground
through diode D2. D2, a 1N4148, adds 0.9 volts offset from ground, and a
TL084 will add from 0.5 to 0.8 volts to this offset. Since a 74HCxx series
part must see less than 1.4 volts (VCC=5.0 v) to sense a logic 0, the
resulting noise margin may be as low as 1.4-(0.8+0.9)= -0.3 volts. This will
cause bit recovery errors and intermittent dropout of DCD (i.e. many retries)
With the TLC274 and 1N3600, noise margin is 1.4-(0.05+0.6)= 0.75 volts - much
better-. Germanium diodes, now somewhat rare, add only 0.35 volts offset and
will boost noise margin even further to about 1.0 volts.
A trade off is that the TL084 slew rate is twice as fast as the TLC274, making
it a faster comparator. However, increased noise margin seems to be the
critical item in my modem.
CONTINUED IN PART 2 OF 4 ...
From : KC6HPN @ WB6YMH.#SOCA.CA.USA.NA
To : 9600 @ ALLUS
Date : 920426/2231
Subject : 9600 MODS PART 2 OF 4
Path : K6VE!WB6YMH
From: KC6HPN @ WB6YMH.#SOCA.CA
TAPR FULL-DUPLEX (NEW) 9600 BAUD MODEM MODS BY KC6HPN (PART 2 OF 4)
2. HOW: Remove capacitor C24 (330 pf) and replace resistor R34 (2.2 k)
with a wire (zero ohm) jumper -BUT ONLY IF YOU PUT IN MOD#3 -
WHY: Capacitance and series resistance are the kiss of death to digital
circuits. Unless U19F is a schmitt-triggered gate, such as the 74HC14, (which
it is not), its output will glitch due to the slow risetime of the input
waveform. Since U19F is in the data path, this will cause an increase in bit
errors, even when the modem is receiving a strong, well formed signal.
Ironically, the RC circuit formed by R34 and C24 may have been intended to
filter out glitches from the bit recovery circuit (U7B, U11D) which has
problems of its own (see mod#3). Or it may have been intended to solve a
critical race condition by providing an RC delay (never use an RC for
this...use a flip-flop instead). In the case of my modem, removing this
circuit once mod#3 was made improved performance.
3. Okay, the easy part is over. This mod is the most critical and the most
difficult. I'm also going to do "WHY" first this time, and "HOW" later.
WHY: It is very, very necessary to add a double-buffering circuit
directly after the data slicer (U4A, D2). A double buffer circuit consists of
two flip-flops connected in SERIES and operated off of the same clock (fig.1).
Its purpose is to synchronize external, asynchronous, signals to an internal
clock without causing glitches.
+5 v
_
|
D o------------------
| |
\ |
10K / |
74HC74 Dual D Flip-Flop \ |
/ |
| |
---------------------------o o---
|10 |4 | | |
----o---- ----o---- | |14 |
12 | PR | 9 2 | PR | 5 | VCC |0.047 uf
A -------| D Q |--------| D Q |---------- B ---
Slicer 11 | | 8 3 | | 6 | Buffered ---
data in ---|>CLK QN|- ---|>CLK QN|- | data out GND |
| | CL | | | CL | | |7 |
| ----o---- | ----o---- | | |
| |13 | |1 | E o---
16x | -----------------o---------- |
clock in | | ---
C ---------------------- -
Figure 1: DOUBLE BUFFER CIRCUIT
CONTINUED IN PART 3 OF 4 ...
From : KC6HPN @ WB6YMH.#SOCA.CA.USA.NA
To : 9600 @ ALLUS
Date : 920426/2234
Subject : 9600 MODS PART 3 OF 4
Path : K6VE!WB6YMH
From: KC6HPN @ WB6YMH.#SOCA.CA
TAPR FULL-DUPLEX (NEW) 9600 BAUD MODEM MODS BY KC6HPN (PART 3 OF 4)
If the input signal to a flip-flop is in transition when the flip-flop is
clocked, the flip-flop will briefly become metastable. That is, its output
will hover between logic 0 and logic 1. In a short period, the flip-flop will
resolve the metastable state and settle to a 1 or a 0. Two identical flip-
flops connected to in PARALLEL to the same asynchronous input (as are U7B and
U16) may not only produce glitches on their outputs, but will often settle to
DIFFERENT logic states. Ooops! Half the modem thinks it sees the bit, half
doesn't. This causes bit errors and faulty DCD.
What the double-buffer does is this: 1. On the first clock edge, the first
flip-flop clocks in the asynchronous data and resolves the metastable state.
2. On the second clock, the second flip-flop clocks in the settled data and
provides a stable, glitch-free output. Propagation delay and setup and hold
time requirements prevent the two flip-flops from clocking the glitchy data
straight through, even though they are using the same clock edge.
A deglitched, synchronous output is then passed to U16 for DCD and bit clock
generation, and to U7B for bit recovery. Glitches formerly generated in U7B
by metastability and passed to U19F through U11D are eliminated, removing the
need for a (bad) glitch filter formed by R34 and C24.
This circuit improved both DCD reliability and data recovery considerably on
my modem. If your DCD indicator (on the modem) flickers even during a strong
signal and you get many retries, this circuit will help.
HOW: The general method is to construct a circuit assembly using an IC
socket and mount the assembly "dead bug" style (upside-down) in the vacant U6
space. If you are using the optional internal clock generator and U6 is
present, use any other convenient location. Once assembled and connected, the
assembly is secured to the modem PC board by double-backed tape.
(A) Obtain a 14-pin PC mount IC socket, preferably with an integral VCC
decoupling capacitor, such as those made by AUGAT. Turn the socket over.
Using fine insulated wire or resistor lead stubs, etc., construct the assembly
as follows:
I. Wire pins 1, 4, 10, and 13 together
II. Wire pins 3 and 11 together
III. Wire pins 2 and 9 together
IV. Solder a 10k resistor between pins 1 and 14
V. If you are not using a decoupled socket, solder a 0.047 uf capacitor
between pins 14 and 7
(B) Once the assembly is completed, place it to one side and examine the modem
board. Using a soldering iron and tweezers, lift the non-banded end of diode
D2 clear of its mounting hole. Next, using the same procedure, lift the end
of R19 nearest to U5, the 7805 voltage regulator. Bend the hanging ends of D2
and R19 together so that they touch.
(C) Using thin, flexible wire, connect point A (pin 12) on the double buffer
assembly to the joined ends of D2 and R19. Secure with solder
(D) Using thin, flexible wire, connect point B (pin 5) on the double buffer
assembly to the mounting hole vacated by R19. The area around the empty R19
hole is crowded, so solder carefully.
CONTINUED IN PART 4 OF 4 ...
From : KC6HPN @ WB6YMH.#SOCA.CA.USA.NA
To : 9600 @ ALLUS
Date : 920426/2236
Subject : 9600 MODS PART 4 OF 4
Path : K6VE!WB6YMH
From: KC6HPN @ WB6YMH.#SOCA.CA
TAPR FULL-DUPLEX (NEW) 9600 BAUD MODEM MODS BY KC6HPN (PART 4 OF 4)
(E) Solder one end of a thin, flexible wire to U18, pin 11 on the back side of
the modem board. Bring the end of the wire up and around to the top of the
board, routing it alongside U18. Connect the wire to point C (pin 11) on the
double buffer assembly.
(F) Next, connect a wire to point E (pin 7), ground, of the double buffer
assembly. Solder the other end of the wire to the center pin of U5, the 7805
voltage regulator.
(G) Connect a wire to point D (pin 14), +5 volts, of the double buffer
assembly. Solder the other end of the wire to the pin of U5 furthest from the
modem PC board edge.
(H) Carefully turn the assembly upright and insert a 74HC74 dual D flip-flop
IC into the socket. Make sure the notch on the IC is on the same side as the
notch on the socket.
(I) Apply a thin strip of double-backed tape to the IC, invert the IC and
assembly and stick it to the modem PC board in the unused U6 location.
(J) Carefully power up the TNC and check for normal operation.
HINTS: Use heat shrink tubing wherever possible to minimize exposed
connections. Carefully premeasure all wire lengths before soldering. Wires
to the assembly should be long enough to allow access to the IC, for insertion
and removal, but no longer.
- END OF TAPR MODEM MODS FILE -
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