Mercury - intermodulation (IMD) tests
HPSDR: Intermodulation Measurements with MERCURYMay/June 2009 / HB9AJG
1. Test Setup
Attenuation between generators: 74.8dB (terminated with 50Ohms)
Attenuation from HP8640B to output: 36.2dB
Attenuation from HP8657A to output: 16.2dB
This test setup was verified with an EK070 RX by Rohde+Schwarz from 80m to 10m, CW, 600Hz bandwidth:
-with 20dB attenuator enabled, MDS was -114dBm
at -13dBm input (each signal) no IM was audible, i.e IM3DR > 101dB
-without attenuator, MDS was -134dBm
at -25dBm input (each signal) IM was audible, i.e IM3DR = 109dB (on 80m, 40m 15m, 10m)
at -30dBm input (each signal) IM was audible, i.e IM3DR = 104dB (on 20m)
2. MERCURY 3rd Order Intermodulation Free Dynamic Range IM3DR
Configuration:Mercury v2.6, Ozy v1.2, Penny v1.1
Dither & Random enabled ( RX sensitivity reduced by 4dB)
Clocks from Penelope
PowerSDR v1.10.4+BaseSVN 2025
PowerSDR RX-Meter calibrated to HP8657A
Bandwidth 48kHz, Buffer size 2048Bits
CWL, 500Hz
Lower frequency = HP8657A
Upper frequency = HP8640B
All dBm readings as displayed on the RX-Meter of PowerSDR (rounded to the next dB)
Signals 3dB above the Noise Floor are considered MDS.
Maximum input signals possible with this Test Setup: -13dBm
* = Artifacts from the ADC (very nonlinearly dependent on the input signals)
**= ADC Overload @ -20dBm
80m: 3680kHz + 3700kHz
Preamp OFF:Noise Floor = -115dBm
Inputs eachdBm-15-20-30-40-50-60-70
IM @ - ∆fdBm-110-----112*-
IM @ +∆fdBm-113-----113*-
IM3DRdB96.552.5
Preamp ON:Noise Floor = -134dBm
Inputs eachdBm-20-25-30-35-40-50-60-70-80-90
IM @ - ∆fdBm-108-125-125-131----134*-132*-
IM @ +∆fdBm-108-128-126-132----134*-132*-
IM3DRdB88101.595.596.56452
40m: 7000kHz + 7020kHz
Preamp OFF:Noise Floor = -115dBm
Inputs eachdBm-15-20-30-40-50-60-70
IM @ - ∆fdBm-109-----114*-
IM @ +∆fdBm-112-----114*-
IM3DRdB95.554
Preamp ON:Noise Floor = -134dBm
Inputs eachdBm-20-25-30-35-40-50-60-70-80-90
IM @ - ∆fdBm-111-127-123-130------
IM @ +∆fdBm-109--126-133------
IM3DRdB9010294.596.5
20m: 14080kHz + 14100kHz
Preamp OFF:Noise Floor = -114dBm
Inputs eachdBm-15-20-30-40-50-60-70
IM @ - ∆fdBm-113------
IM @ +∆fdBm-114------
IM3DRdB98.5
Preamp ON:Noise Floor = -134dBm
Inputs eachdBm-21**-25-30-35-40-50-60-70-80-90
IM @ - ∆fdBm-116-125-126-131------
IM @ +∆fdBm-112-125-123-134------
IM3DRdB9310094.597.5
15m: 21080kHz + 21100kHz
Preamp OFF:Noise Floor = -114dBm
Inputs eachdBm-15-20-30-40-50-60-70
IM @ - ∆f-107-113*-114*-113*-113*-112*-
IM @ +∆fdBm-106-114*-114*-114*-113*-112*-
IM3DRdB91.593.58473.56352
Preamp ON:Noise Floor = -133dBm
Inputs eachdBm-21**-25-30-35-40-50-60-70-80-90
IM @ - ∆f-104-115-122-129-133*-133*-132*-132*-
IM @ +∆fdBm-103-116-120-130-132*-131*-131*-
IM3DRdB82.590.59194.5-9382.571.561.5
10m: 28080kHz + 28100kHz
Preamp OFF:Noise Floor = -113dBm
Inputs eachdBm-15-20-30-40-50-60-70
IM @ - ∆f-107-113*-113*-113*-111*-110*-
IM @ +∆fdBm-107-113*-113*-112*-111*-110*-
IM3DRdB92938372.56150
Preamp ON:Noise Floor = -134dBm
Inputs eachdBm-21**-25-30-35-40-50-60-70-80-90
IM @ - ∆f-96-108-120-131-131*-128*-128*-128*-128*-
IM @ +∆fdBm-95-107-120-134-134*-131*-131*-129*-129*-
IM3DRdB74.582.59097.592.579.569.558.548.5
6m: 49080kHz + 49100kHz
Preamp OFF:Noise Floor = -113dBm
Inputs eachdBm-15-20-30-40-50-60-70
IM @ - ∆f-97-111-----
IM @ +∆fdBm-95-107-----
IM3DRdB8189
Preamp ON:Noise Floor = -133dBm
Inputs eachdBm-21**-25-30-35-40-50-60-70-80-90
IM @ - ∆f-76-89-104-120-131-130*-132*-132*--
IM @ +∆fdBm-75-88-103-118-127-130*-132*-132*--
IM3DRdB54.563.573.58489807262
3. Effect of "Dither" and Random"August 2009 / HB9AJG
Even slight imperfections of the ADC transfer curve will result in unwanted signals. The Dither signal randomizes the location of the input signal on the ADC transfer curve, resulting in an improved IMD performance. Part of the Dither signal will leak through to the output, causing a rise of the noise floor.
Due to the small size of the ADC, and depending on the PCB layout, coupling from the output to the input may occur. The Random signal is to prevent such interferences from appearing as regular spectral lines. So it would not be expected to have much effect on the IMD performance.
The tests below show that both signals are required to achieve the best IM3DR.
IM3 signals 3db above the noise floor are considered MDS.
80m: 3680kHz + 3700kHz, Preamp OFF
3.1Dither enabled, Random enabled: Noise Floor = -115dBm
Inputs eachdBm-15-20-30-40-50-60-70
IM @ - ∆fdBm-110-----112-
IM @ +∆fdBm-112-----112-
IM3DRdB9652
3.2Dither enabled, Random disabled: Noise Floor = -115dBm
Inputs eachdBm-15-20-30-40-50-60-70
IM @ - ∆fdBm-102-102-99-99-98-98-110
IM @ +∆fdBm-103-104-99-100-98-97-110
IM3DRdB87.5836959.54837.540
3.3Dither disabled, Random enabled: Noise Floor = -118dBm
Inputs eachdBm-15-20-30-40-50-60-70
IM @ - ∆fdBm-106-110-98-102-112-113-
IM @ +∆fdBm-110-107-98-102-110-114-
IM3DRdB9388.568626153.5
3.4Dither disabled, Random disabled: Noise Floor = -118dBm
Inputs eachdBm-15-20-30-40-50-60-70
IM @ - ∆fdBm-101-105-94-97-100-113-116
IM @ +∆fdBm-104-106-95-97-101-114-116
IM3DRdB87.585.564.55750.553.546
20m: 14080kHz + 14100kHz, Preamp OFF
3.5Dither enabled, Random enabled: Noise Floor = -114dBm
Inputs eachdBm-15-20-30-40-50-60-70
IM @ - ∆fdBm-113-----112-
IM @ +∆fdBm-114-----113-
IM3DRdB98.552.5
3.6Dither enabled, Random disabled: Noise Floor = -115dBm
Inputs eachdBm-15-20-30-40-50-60-70
IM @ - ∆fdBm-96-96-92-90-92-99-114
IM @ +∆fdBm-98-97-95-92-93-100-114
IM3DRdB8276.563.55142.539.544
3.7Dither disabled, Random enabled: Noise Floor = -117dBm
Inputs eachdBm-15-20-30-40-50-60-70
IM @ - ∆fdBm-105-114-96-100-111-115-
IM @ +∆fdBm-108-111-97-99-110-114-
IM3DRdB91.592.566.559.560.554.5
3.8Dither disabled, Random disabled: Noise Floor = -118dBm
Inputs eachdBm-15-20-30-40-50-60-70
IM @ - ∆fdBm-97-94-91-91-95-100-116
IM @ +∆fdBm-96-98-94-92-95-100-116
IM3DRdB81.57662.551.5454046
4. MERCURY IM3DR with a third signal presentMay/June / HB9AJG
In his presentation on the design of MERCURY at the ARRL/TAPR Conference on Digital Communications 2008 Phil Harman explains that other strong signals have the same effect as "Dither enabled". To verify this, a second coupler and a third signal generator were added to the Test Setup:
"Dither" and "Random" were disabled, and the measurements on 20m were repeated:
Preamp OFF:Noise Floor = -118dBm
Inputs eachdBm -15-20-30-40-50-60-70
IM @ - ∆fdBm -97-94-91-91-95-100-116
IM @ +∆fdBm -96-98-94-92-95-100-116
IM3DRdB4046
Third signal -52dBm*
IM @ - ∆fdBm -94-91-92-100-114*-
IM @ +∆fdBm -98-94-92-100-114*-
Third signal -47dBm*
IM @ - ∆fdBm -94-92-93-114*--
IM @ +∆fdBm -98-93-94-114*--
Third signal -14dBm*
IM @ - ∆fdBm -112-112-114*---
IM @ +∆fdBm -112-112-114*---
IM3DRdB74
Frequency of the third signal: anywhere between 1 and 30MHz (except harmonics and IMs)
* Approximate signal level, the limit is fuzzy: at levels up to about 10dB higher IM signals vary between nil and MDS
With very strong third signals (S9+60dB), an improvement of the IM3DR of approximately 30db is achieved.
However, this is not as much as with "Dither" and "Random" enabled (copy from page 2):
Preamp OFF:Noise Floor = -114dBm
Inputs eachdBm-15-20-30-40-50-60-70
IM @ - ∆fdBm-113------
IM @ +∆fdBm-114------
IM3DRdB98.5
Now, of course, the next step was to connect my DX-88 8-band trap vertical instead of the third signal generator (no filters between antenna and Mercury):
BandPreampNoise floor UTCmax gen levelCommentIM3DR
for IM3 <MDS
20mOFF-114dBm15:30-50 dBmvaries slightly with time64dB
ON-115-41(fading effects?)74
40mOFF-108dBm16:00-40 dBmvaries slightly68
ON-108-2484
80mOFF-97dBm16:10-34 dBmvaries slightly63
ON-97-22just before ADC Overload!75
15mOFF-114dBm16:20-46 dBmvaries by several dB68
ON-116-4076
10mOFF-116dBm16:30-47 dBmvaries by several dB69
ON-123-4578
6mOFF-118dBm16:40-46 dBmvaries slightly72
ON-136-5086
CONCLUSION: The antenna has about the same effect as a third signal generator. Enabling "Dither" and "Random" results in a much higher IM3DR, however. Therefore, I think if a band is heavily congested, it is preferable to enable "Dither" and "Random" and to accept the degradation of sensitivity of 4dB.
