"An Octal 10 MHz Distribution Amplifier".

"An Octal 10 MHz Distribution Amplifier".
An Octal 10 MHz Distribution Amplifier
W.J. Riley
Hamilton Technical Services
Beaufort, SC 29907 USA
 Introduction
This paper describes an octal 10 MHz distribution
amplifier module that provides eight isolated +7
dBm nominal outputs from a rubidium oscillator or
similar frequency source, as shown in Figure 1. It
comprises an input buffer and level detector, RF
power splitting and eight output amplifiers, along
with a DC/DC converter operating from a +24
VDC power supply. The front panel contains the
eight BNC outputs (see Figure 2), while the rear
panel holds the 5.5 mm OD x 2.5 mm ID DC power
and BNC RF input connectors along with their
respective indicators (see Figure 3).
Figure 1. Octal 10 MHz Distribution Amplifier
Figure 2. Front Panel
Figure 3. Rear Panel
 Description
A block diagram of the octal 10 MHz distribution
amplifier is shown in Figure 4. The RF input is
terminated with a 50  resistor and the eight
output amplifiers have 50  resistors at their
outputs, thereby providing good input and output
return loss. The internal input buffer, power
splitter and O/P amplifier paths also have 50 
interface impedances. The nine amplifiers all use
LMH6723 wideband amplifiers. The input level
detector provides an indication that an acceptable
input signal is applied. The modular DC/DC
converter has additional filtering at its output to
further suppress ripple. Although indended for
10 MHz, this distribution amplifier will work
satisfactorily from 1 to 25 MHz.
O/P
Amplifiers
Level
Detector
Power
Splitter
10 MHz
+7 dBm
50 
1
2
10 MHz
+7 dBm
50 
3
Input
Buffer
4

5
Power
+24
VDC
DC/DC
Converter
6
+5V
-5V
7
8
Figure 4. Distribution Amplifier Block Diagram
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 Circuit
Schematics of the octal 10 MHz distribution amplifier are shown in Figures 5-8. The +7 dBm nominal 10
MHz input is buffered by a 3.5 dB gain 50  input and output RF amplifier that drives a 2-way power
splitter whose outputs each drive four output 0 dB gain 50  input and output amplifier sections. The
overall configuration thus has nominal unity gain from the RF input to each output. The power section
comprises a well-filtered ±5 volt DC/DC converter that operates over an input range of +18 to +36 VDC
(+24 VDC nominal).
Figure 5. Input Section
Figure 6. Output Amplifier 1-4 Section
Figure 7. Output Amplifier 5-8 Section
Figure 8. Power Section
 Board Layout
The 3.8” x 2.5” board layout of the octal 10 MHz distribution amplifier is shown in Figures 9 and 10.
The board is double-sided with a nearly full ground plane on the bottom. All components are SMD type
on the top side of the board. Besides the nine SMB RF connectors, there is a terminal strip for the DC
input and a 2-position 0.1” header for the input level LED indicator. There are test points for the +5V and
-5 volt supplies, and two ground TPs. Wire leads and an in-line connector are used for the LED to
preserve a low profile. The board is attached to the case with four mounting screws.
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Figure 9. PWB Layout
Figure 10. PWB Silk Screen
A photograph of the assembled distribution board is shown in Figure 11 and the complete unit in its initial
test configuration is shown in Figure 12.
Figure 11. Assembled Distribution Amp Board
Figure 12. Initial Test Configuration
 Panel Assemblies
The front and rear panel assemblies of the octal 10 MHz distribution amplifier are shown in Figures 13
and 14 respectively.
Figure 13 Front Panel Assembly
Figure 14 Rear Panel Assembly
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The front panel has eight 12” right angle SMB plug to bulkhead BNC receptacle RG-178 cables for the
RF output signals. The rear panel has one of these cables for the RF input signal, a Switchcraft Type
712A 5.5 mm OD x 2.5 mm ID DC connector and two panel mounted LEDs.
 Instrument Packaging
The octal 10 MHz distribution amplifier is housed
in an 8.66” long x 6.50” wide x 1.20” high
Hammond 1455R instrument case. This case was
chosen as having the minimum width for the eight
BNC RF output connectors and the minimum
height to fit the board and its right angle SMB RG
connectors. An interior view of the packaged unit
is shown in Figure 15.
Figure 15. Interior View of Distribution Amp
 Output Power
The octal 10 MHz distribution amplifier has a nominal unity gain (0 dB). When driven at 10 MHz at
+7.00 dBm, the output power at the eight outputs was measured as shown in the table below. The second
bank appears to have about 0.1 dB higher gain, but all are outputs are close to nominal. There is no
dependence of RF output power on the DC supply voltage, and the gain is essentially flat from 1 to 50
MHz. With a 50  load, the output clips at 2 volts peak-to-peak (4 volts p-p no load) at a drive and
output level of about +11 dBm.
Channel
O/P, dBm
1
+7.09
2
+7.10
3
+7.06
4
+7.09
5
+7.15
6
+7.19
7
+7.19
8
+7.19
 Input Level Detector
The input level detector indicates the presence of an input signal at a drive power of ≥ +5.3 dBm.
Operation at lower drive level is, however, fine. The output saturates at a drive level of about +11 dBm.
 Input and Output Return Loss
The measured reference input return loss was 33 dB at 10 MHz, and ≥ 26 dB at all frequencies above 3.8
MHz, as shown in Figure 16. The measured RF output #1 return loss was 47 dB at 10 MHz, , and ≥ 26
dB at all frequencies between 3.4 MHz and 45 MHz, as shown in Figure 17. The RF output return loss is
essentially the same for all channels.
4
Figure 16. RF Input Return Loss vs Frequency
Figure 17. RF O/P #1 Return Loss vs Frequency
 Forward Gain
The forward gain of the distribution amplifier is nearly constant at 0 dB between 1 and 100 MHz, as
shown in Figure 18. It is +0.24 dB at 10 MHz and within ±0.5 dB between 1 MHz and 44 MHz. The
gain is essentially the same for all channels.
 Reverse Isolation
The reverse isolation between an output and the input of the distribution amplifier is 57 dB at 10 MHz and
greater than 38 dB at any frequency between 1 and 50 MHz, as shown in Figure 19. The reverse isolation
is essentially the same for all channels.
Figure 18. Forward Gain vs Frequency
Figure 19. Reverse Isolation vs Frequency
 Interchannel Isolation
Interchannel isolation (the isolation between one output and any other output) is an important attribute of
a multichannel distribution amplifier. In this design, channels 1-4 and 5-8 are driven from different
outputs of a 2-way RF power splitter (see Figure 4) so one might expect better isolation between outputs
in different banks than in the same bank. The reverse isolation at 10 MHz from either outputs 2-4 or
outputs 6-8 to output #1 was > 75 dB (the noise floor of the measurement). At 100 MHz, the isolation
5
from outputs 2-4 became about 60 dB, while remaining at or below 70 db for outputs 6-8. The actual 10
MHz interchannel isolation may be even better.
Another aspect of this parameter is the amplitude change at one output when the load changes at any other
output (e.g., if another output is shorted). No such effect was measured to a resolution of ≤ 0.05 dB.
 Phase Noise
The additive phase noise of a typical distribution amplifier channel is shown in Figure 17. While this
level of performance isn’t exceptionally good, it is quite adequate for its intended purpose to buffer a
rubidium oscillator whose phase noise is significantly higher everywhere.
Sideband Frequency
Hz
1
10
100
1k
10k
100k
Phase Noise
dBc/Hz
-125
-138
-149
-150
-153
-155
Figure 17. Distribution Amp Phase Noise
 Spectral Purity
The spectral purity of a typical distribution amplifier channel is shown in Figure 21. All channels are
essentially the same, and clean within the resolution of the spectrum analyzer. No spurious components
are visible with any frequency span. Power line and other low-level spurs did appear in the phase noise
measurement but were suppressed in Figure 17.
6
 Harmonic Distortion
The harmonic distortion of a typical distribution amplifier channel is shown in Figure 22. All channels
are essentially the same, and, since the unit is driven from a source having much less distortion, these
results are believed to be those of the distribution amplifier. The largest harmonic is the 3 rd at a relative
level of about -38 dBc.
Figure 21. Carrier Spectral Purity
Figure 22. Harmonic Spectrum
 Power Supply
The octal 10 MHz distribution amplifier can be powered by the same +24 volt, 1 ampere power supply
that runs an Efratom LPRO rubidium oscillator. The input power is applied via a 5.5mm OD x 2.5 mm
ID DC power plug. The unit has a series diode in its DC input line that provides reverse polarity
protection.
 DC Power Consumption
The octal 10 MHz distribution amplifier draws about 30 mA at 24 VDC without RF loads and 47 mA
(1.13 watts) fully loaded. Because of the DC/DC converter, the power consumption is fairly constant
versus input voltage.
 Operation
The power LED will illuminate when +24 VDC power is applied to the unit, and the RF input will
illuminate when a signal of ≥ +5 dBm is connected to the rear RF input. The eight RF outputs will then
be present at the same level as the input.
 Applications
Applications for this instrument include its use to distribute the 10 MHz output of a frequency standard
(such as a rubidium oscillator) to other laboratory instruments (e.g., counters, synthesizers, spectrum
analyzers and frequency measurement systems). The distribution amplifier is broadband, and can be used
at any frequency between 1 and 25 MHz.
7
 Specifications
Preliminary specifications for the Octal 10 MHz distribution amplifier are as follows:
Parameter
Specification
Remarks
RF Input Power
+7 dBm nominal sinewave into 50 
RF Input Impedance
RF Input Connector
RF Input Level Detector
RF Output Power
Frequency Range
Waveform
RF Output Power (each
output)
RF Output Impedance
RF Output Connectors
Interchannel Isolation
Output – Input Isolation
Phase Noise
50  ±5%
BNC receptacle
+5 dBm nominal threshold
+7 dBm nominal sinewave into 50 
1 to 25 MHz (10 MHz nominal)
Sinusoidal, no amplitude limiting
+7 dBm nominal sinewave into 50 
The gain saturates at an input level of
about +11 dBm.
Return Loss ≥ 26 dB.
Rear panel.
Lower drive level is OK.
Unity gain to all outputs.
Can be used up to 50 MHz.
Wideband, no low pass filtration.
Output voltage is x2 into open circuit.
Outputs are short circuit proof.
Return Loss ≥26 dB.
Front panel with ¾ inch spacing.
Between any two outputs at 10 MHz.
Between any output and RF input.
Typical
Size (HxWxD)
50  ±5%
Eight BNC receptacles
≥70 dB
≥55 dB
SB Freq, Hz Phase Noise, dBc/Hz
1
-125
10
-138
100
-149
1k
-150
10k
-153
100k
-155
0 to +50 °C
All ≤ -35 dBc
All ≤ -70 dBc
TBD
+24 VDC nominal at 50 mA DC full
load, 30 mA no load.
1.20” x 6.50” x 8.66”
Weight
Availability
1.8 lbs. (835 grams)
Not for sale
Operating Temperature
Harmonics
Non-Harmonics
TC of Phase
DC Power
Specs apply at room temperature.
At +7 dBm output into 50  load.
Spurii.
Input voltage range +18 to +28 VDC.
Excluding connectors and feet.
Approximately 1.5” high with feet.
Excluding cables.
Design information available at no
cost upon request.
 References
None
File: An Octal 10 MHz Distribution Amplifier.doc
W.J. Riley
Hamilton Technical Services
September 3, 2011
Revision A June 8, 2012
8
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