Mercury - intermodulation (IMD) tests

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HPSDR: Intermodulation Measurements with MERCURYMay/June 2009 / HB9AJG

Test Setup

MERCURY IM3DR 1.gif

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)


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

==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

==MERCURY IM3DR with a third signal present== (May/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:

MERCURY IM3DR 2.gif

"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.