Tucson TADD-1 Installation and operating instructions

TADD-1 Assembly and Operation Manual
Six Channel RF Distribution Amplifier
Revised 5 April 2014
©2005-14 Tucson Amateur Packet Radio Corporation
Introduction
NOTE: This is an update to the 2007 version of the manual that collects and hopefully corrects some
additional information (including the change from the MA 477 to the AD8055 amplifier chip), and
also provides some new performance information.
The TADD-1 is a six channel RF distribution amplifier. Its primary purpose is to allow one frequency source
(typically a frequency standard) to drive several loads (typically the "external reference" input of frequency
counters or other test equipment) with high isolation. The input frequency can range from 500 kHz to 30
MHz.
While the signal from a frequency standard is normally in the range of 0 to +13 dBm, an adjustable gain
control allows the TADD-1 to be used with a wide range of input signal levels. The maximum output signal
is approximately +12.7 dBm into 50 ohms (2.75 volts peak-to-peak, or 0.97 volt RMS). Maximum output
voltage into a high impedance load is slightly higher. A jumper allows the input signal to be terminated in 50
ohms, or to operate at high impedance (approximately 47 kohms).
Each RF input and output on the TADD-1 is transformer coupled to minimize the possibility of ground loops.
In addition, the connectors are capacitively coupled to ground to further improve isolation (and allow
operation with signals riding on DC bias).
The TADD-1 includes a fused and reverse-polarity protected 9 volt regulator. The supply voltage can range
from 11.5 to 24 volts. Current drain will depend on the output loads, but is typically in the range of 60 to 150
milliamps.
See the "Performance" section for further details.

Circuit Description
The TADD-1 uses the Maxim MAX477 Analog Devices AD8055 amplifier (please read all “MAX477”
references in this document as “AD8055”). The MAX477 DIP package version has been discontinued. The
AD8055 is a drop-in replacement with nearly identical performance; in fact, limited testing has shown that is
has a few dB lower phase noise than the MAX477 device.
The MAX477 is a video amplifier designed to drive coax lines, so it is well suited to this application. It has a
maximum operating frequency of 300MHz, and this requires care in the circuit design and layout to avoid
oscillation and spurious outputs.
The input signal enters via J1 and T1. C11 provides an RF ground reference while maintaining DC isolation.
JP1 and JP2 are headers that, if installed, allow several TADD-1 boards to be stacked and share a common
RF input. JP3 allows 51 ohm resistor R12 to terminate the input signal if desired.
The MAX477 output level is internally clamped to a maximum of 7 volts peak-to-peak (5 volts Pk-Pk when
loaded by 50 ohms). Since IC1 has a minimum gain of 2, it's possible that a strong input signal could
overdrive the amplifier and result in output clipping. R4 and R6 form a voltage divider that can be used to
reduce the input signal level if necessary. The default values (R4=10k and R6=100k) result in minimal
attenuation. Changing their ratio allows greater attenuation. For example, using 47k for both resistors will
reduce the input voltage by half. Given that the amplifier has a gain of up to 13, it's best to err on the side of
increased attenuation if your input signal level is robust.
The voltage divider described in the deleted text above provided excess attenuation, and limited the TADD-1
to less than unity gain. In current units, R4 has been changed to 1 kohm and R6 has been eliminated. In
addition, R1 and R8 have been changed from 15 kohms to 47 kohms. These changes substantially increase
the maximum gain. All TADD-1 units shipped after September 15, 2006 include these changed components,
and all earlier units may be retrofitted by changing the values of R4, R1, and R8 as noted, and removing R6.
No board changes are required.
The TADD-1 operates from a single 9 volt regulated power supply, so input signals to the MAX477
amplifiers must ride on a 4.5 volt DC bias. R1 and R8 provide that bias for the input of IC1, and C8 provides
DC isolation.
Trimmer R13 adjusts the gain of IC1. The gain of a non-inverting op amp is equal to 1+Rb/Ra. In the
TADD-1, Ra is R10 (220) plus R13 (0 to 2.5 kohm) and Rb is R11 (2.7 kohm). Therefore, the gain ranges
from about 2 (when R13 is set to 2.5 kohm) to about 13 (when R13 is set to 0 ohms). Because our circuit
uses a single supply voltage, Rb is referenced to 1/2 the supply voltage rather than ground. R14 and R15
provide that reference. The value of R14 will also have some effect on the gain, so relatively low values (560
ohms) are used for this voltage divider. As a result, the full gain of 13 can't be reached; using lower value
resistors at R14 and R15 permit higher gain, at the expense of greater power consumption.
Wire jumpers J2, J4, and J5 allow an optional bandpass filter to be added to the circuit. Normally, only J2 is
installed, which bypasses the filter. Installing J4 and J5, and leaving J2 empty, put the filter in-line. The
filter is described later in this manual.
The output of IC1 drives IC2 through IC7. The RF coming out of IC1 rides on a DC bias equal to half the
supply voltage, and that bias is used on the input to the following amplifiers. This avoids the need for
additional biasing components on the inputs of IC2-7.
If you observe the board layout, you'll see that the traces connecting to the inputs of IC2-7 are laid out in a
serpentine fashion. This equalizes the lead length driving each amplifier, minimizing any difference in signal
phase at the outputs. However, the inductance of those long leads may cause the high-bandwidth MAX477

to break into parasitic oscillations. R9 provides a low impedance load at the beginning of the "serpentine"
and helps to stabilize IC1. It also provides a load for the bandpass filter if that is used. C12 prevents the DC
bias from being grounded through the resistor. The 100 ohm resistors at the input to each of IC2 through IC7
decouple the amplifier from the long traces and are primarily responsible for stabilizing the system.
R9 was originally 51 ohms, but increasing that to 270 ohms allows somewhat higher output signal levels,
and makes a significant difference in output if the optional bandpass filter is used. Increasing the value of
this resistor does not seem to have an impact on stability, but in the event of parasitic oscillation, reverting to
a lower value may help.
IC2-7 are configured as unity gain followers. Their outputs are DC isolated, fed through a “back
termination” into an isolation transformer. As on the input, the connector is grounded through a capacitor to
further reduce the possibility of ground loops,. The back termination is a 50 ohm series resistor designed
both to match the coax cable impedance, and to ensure amplifier stability even when driving a capacitve load
(such as a long coax cable). Because the back termination and the load impedance form a voltage divider, the
output voltage into a 50 ohm load will be half the unloaded voltage. The adjustable gain of IC1 compensates
for this loss (within reason).
The power supply uses a standard LM7809 regulator. JP5 and JP6 are provided for board-stacking purposes.
F1 is a 0.5 amp pico-fuse. C39 and R27 form an input ripple filter. D1 provides reverse voltage protection;
if the power leads are reversed, D1 will conduct and blow F1. To help stability and minimize noise, lots of
bypass caps are used -- each MAX477 has a 0.1 and 0.001 cap at its supply input, and additional 1uF
tantalum caps are spaced along the power bus.

OPTIONAL BANDPASS FILTER
NOTE We do not recommend using the filter unless it is required by the application. Any tuned circuit will
produce phase shifts that vary with temperature, and this creates an additional source of instability.
Also note that the earlier version of this document had some erroneous information about component values
and toroid winding. That has been partially corrected in the assembly instructions, but the frequency
response plot below is from the original filter testing and does not reflect the slightly changed values
recommended now. And, the schematic has not been updated to show the changed values.
The optional bandpass filter (“BPF”) is laid out on the circuit board as a 2nd order series-input filter. The
component values suggested in the schematic (and shown in the assembly instructions below) implement a
Chebycheff filter with a center frequency of 10 MHz, though other designs can be used. Its rejection is
greater than 30 dB at 5MHz (one-half the fundamental), and is 35 dB or more at 20 MHz (the second
harmonic) and above. See the frequency response plot below for more performance details.
This plot is based on a 270 ohm value at R9, which terminates the filter. The value of this resistor has an
impact on both the output level and the frequency response of the filter. Using a 50 ohm resistor will result in
as much as 3 dB better stopband attenuation, but increases the insertion loss by almost 6 dB.
The assembly instructions include component values for center frequencies of both 5 MHz and 10 MHz.

TYPICAL PERFORMANCE

PHASE NOISE
This plot shows the residual phase noise of TADD-1 units. The blue trace is of a unit built with the MAX477
chips, and the magenta trace is a unit built with the AD8055.
REVERSE ISOLATION
Isolation from an output channel to the input, and from output-to-output is typically greater than 80dB at 5
MHz and 10 MHz(which may be instrumentation-limited). Isolation decreases below 5 MHz.

PHASE DELAY ACROSS CHANNELS
This plot overlays the phase delay at 5 and 10 MHz of each of the six channels in a unit built without the
bandpass filter. Channels 1 through 3 form a tight group with almost identical phase through the amplifier.
Channels 4 through 6 show slightly higher delay (about 0.75 degrees at 5 MHz; much less at 10 MHz. I am
not sure what causes this slight discrepancy; the good news is that the phase across all channels is consistent
to well within 1 degree.

Typical Performance
Current Draw, 13.8V*
Condition (50 Ohm Loads) Current
No Input, No Output 60ma
RF In, No Output 89ma
1 output 96ma
2 outputs 103ma
3 outputs 110ma
4 outputs 117ma
5 outputs 124ma
6 outputs 131ma
* Current will vary with input level and gain setting
Gain and Signal Levels (10MHz)
( NOTE: This data is based on the original design and has not been retested after circuit
changes.)
Maximum input for no clipping at maximum gain +3.5dBm terminated in 50 ohms.
Gain range with +3.5dBm input -0.8 to +7.7 dB (output terminated in 50 ohms)
Maximum output level into 50 ohms +12.7dBm
Maximum output level into high impedance 5.12V peak-peak
Harmonics*
Source (+6.7dBm) Minimum Gain Maximum Gain
Second Harmonic -46 -45 -47
Third Harmonic -54 -47 -46
Fourth Harmonic -61 -55 -55
* This table is suspect because the gradually increasing gain curve of the TADD-1 through 40MHz will
amplify the source harmonics more than the fundamental.

Preparation
The TADD-1 kit includes the printed circuit board, a bag with seven BNC connectors, and several manila
envelopes containing small components. Parts that are used in quantity are in separate envelopes, labeled
with the component each contains. One or more envelopes labeled “Misc.” contain parts that are used in
small quantities. Integrated circuits and transformers may be stuck to anti-static foam, or be placed in plastic
tubes. The 7809 voltage regulator will be in one of the “Misc.” envelopes. Check to make sure your kit
includes all these bits, and check the contents of the envelopes against the parts inventory below. If anything
is missing, contact the TAPR office.
Refer to the layout diagram for clarification of parts placement. All references to up, down, left, and right
assume that you are looking at the PCB with the "TADD-1" text and copyright notice along the right-hand
side of the board. All components are mounted on the top of the PC board.
If you plan to mount the TADD-1 using the four corner holes, now is a good time to use the PC board as a
template for marking the mounting holes in your enclosure.
Check your soldering iron to be sure the tip is in good condition. The tip should be the small conical tip type
and must be clean. If you can't remember when you last replaced the tip, now would be a good time to do so.
All parts should be mounted as nearly flush to the board surface as practical without stressing the lead.

Parts Inventory
OK? Qty Value Part
8 0.001uF C5, C6, C7, C8, C18, C19, C33, C34
7 0.01uF C11, C13, C14, C25, C26, C37, C38
18 0.1uF C1, C2, C3, C4, C9, C10, C12, C15, C16, C17, C20, C21,
C22, C29, C30, C35, C36, C41
1 0.33uF C40
3 1uF C42, C43, C44
1 10uF C39
1 SA15A TVS D1
1 0.5A PICOFUSE F1
7MAX477
AD8055 IC1, IC2, IC3, IC4, IC5, IC6, IC7
1 LM7809 IC8
7 BNC J1, J3, J6, J7, J8, J9, J10
7 Washer and Nut --
12-pin header and
shorting block JP3
12-pin Molex
header JP5
12-pin Molex
connector --
2 Molex crimp pins --
1 1 R27
8 51 R5, R7, R12, R18, R19, R21, R25, R26
6 100 R2, R3, R16, R17, R23, R24
1 220 R10
1 270 R9
2 560 R14, R15
1 2.5k POT R13
1 2.7k R11
1 10k 1k R4
2 15k 47k R1, R8
1 100k R6 DELETED
7 T1-1-X65 T1, T2, T3, T4, T5, T6, T7