Digital Radio SeminarDigital Radio SeminarAM HD Radio

Jeff WeltonJeff WeltonRegional Sales Manager Regional Sales Manager –– Central U.S.Central U.S.WBA B d t ’ Cli iWBA B d t ’ Cli iWBA Broadcasters’ ClinicWBA Broadcasters’ ClinicMadison, WI October 2008Madison, WI October 2008

© Nautel Limited 2006This presentation has been produced for Nautel customers and agents and is not for distribution without the express written consent of Nautel.

Typical Setup

Magnitude

Carrier Frequency andPhase Information GPS

Antenna

STLAESSource

Matching &

Sync'd AES

atc g &Phase Delay

AES

y

AES (ANALOG)Audio Processor

Transmitter RF Load Requirements

Step 1 – Determine the RF load characteristics

• What does the antenna plot look like?• What does this load look like to the RF amplifiers?What does this load look like to the RF amplifiers?• What is the delay between the antenna and RF amplifiers?

ANTENNA VSWR

• What does the antenna plot look like over Fc+/- 15 kHz?• Limits are 1.4:1 at Fc+/- 15 kHzLimits are 1.4:1 at Fc / 15 kHz• Plot should be symmetrical

Antenna VSWR

Smith Chart h i 1 4 1showing 1.4:1

VSWR Circle

1.4:1 VSWR

Smith Chart showing S t i l l da Symmetrical load

plot.

This is ideal load for amplifiers….not necessarily the same for the output of thefor the output of the transmitter!

Delay through the Transmitter

• How do we determine delay through the transmitter?• Nautel WEB site – www.nautel.comNautel WEB site www.nautel.com• Measure actual delay

www.nautel.com

Marketing 2006 g

www.nautel.com

Phase Delay Measurement

PA VOLTSC2

E1

C40.01

5J1

Q52N4401R3

15K####R4

R5

CR5

# RF ALARM ?

C20.047

BROWN3T

Q1STW14NM50

T1

PA VOLTSSAMPLE

RF (1)C51.0

E2

7J12W

R533K

1000R6

1000R7

CR1MUR460

RF DRIVE

1 J1

3 J1

2 J1

4 J1

6 J1

47.5R1

1 5

U1:A

C30.47

15W2.5R2

GREEN

RED3T

YELLOW 2T

ORANGE 2T

Q3STW14NM50

Q2STW14NM50 C6

1.0CR2MUR460

CR3MUR460

+15VC11.0

F11A

SLOW

9 J1

8 J1

10 J1 GREEN3T

BLUE3T

4GND

2GND3 Vs U1:BMIC4452CT

Q4STW14NM50

RF (2)E3

CR4MUR460

JOB07\PA\197\1056S-A SHEET 1 OF 1 VD

Measure output of RF amplifier and transmitter RF output

Phase Delay Measurement

RF Amplifer Voltage

Rf OutputRf Output

Phase Delay Correction

Smith Chart showing S t i l l da Symmetrical load

plot. Let’s assume this load characteristic at the transmitter RF Output

Phase Delay Correction

Example for 690 kHz

• 152 degrees delay through RF filterg

• Plot shows desired load for output of transmitter

Toward load

Phase Delay Measurement

Toward load

Example for 690 kHz

• 28 degrees required between qdesired and current antenna plot

Phase Delay Measurement

Delay Circuit Design

Design Pi section for 28 degree delay

Delay of ½ Pi section (14 degrees) is = tan-1 (n-1) .5, where n is z transform

n=1.062

+j

+j = 50 (n-1) .5 = +j11.723

n+j

-j

n

-j = 50 = -j200

(n-1) .5

Delay Circuit Design

Design Pi section for 28 degree delay

Complete Pi with two LC sections

5 408uH

+j 11.723 * 2 = +j 23.446 = 5.408 uH @ 690 kHz

-j200 = 1149 pF @ 690 kHz5.408uH j200 1149 pF @ 690 kHz

1149 pF 1149 pF

Fine Tuning Options

Transmitter operation can be optimized with pre-correctionTransmitter operation can be optimized with pre correction.

• Measure the transmitter amplitude response into antenna• Measure transmitter RF Drive phase response into antennaMeasure transmitter RF Drive phase response into antenna• Nautel provide pre-correction data for NE IBOC

• Newer models have pre-correction designed inp g

Optimize performance

Transmitter amplitude d i t 50response measured into 50

ohm load

Pre-correction response createdcreated

Transmitter Phase response measured into 50 ohm loadmeasured into 50 ohm load

Pre-correction response created

Spectrum shows excellent reduction in Intermodulation re-growthgrowth

Antenna optimization

Transmitter amplitude d i tresponse measured into

antenna

Pre-correction response createdcreated

Transmitter Phase response measured into antennameasured into antenna

Pre-correction response created

Spectrum of transmitterSpectrum of transmitter into antenna model at Nautel showing a good margin of safety with

f t threference to the emissions mask

On Air Performance

Spectrum of transmitterSpectrum of transmitter into actual antenna showing a satisfactory margin of safety with

f t threference to the emissions mask

Spectrum using loop antennap g p

FPGA Block Diagram

ateEqualizer B+ 9 Phase PDMAM/AMI Q to Mag 8x FIR 32x CIC

9 PDMGaq

FIR – 64 Tap Comp Generator

I,Q Ref310 kSPS

Correctione I Q t M AM/PM M Ph

I,Q to Mag Interpolator Interpolator

256 I t RF Ph M d l t d

9 PDM 317

MSPSG

ate I,Q to Mag

PhaseAM/PM

CorrectionMag Phase

To I RF DAC

Carrier FrequencyComplex Oscillator

256x InterpFIR/CIC

RF Phase Modulated Carrier 79.4 MSPS

ReflectedPowerSense

2fc IIRFilter

High RFCurrentSenseG

ate256x Decimate

FIR/CICScale &Phase

Correction

RF ADCCombiner Current

Combiner Current310 kSPS

2fc IIRFilter

Sense

Gat

e256x DecimateFIR/CIC

Correction

RF ADCCombiner Voltage

Combiner Voltage310 kSPS

Pre-Correction Principle

An amplifier characteristic g(x)may be corrected for with a complementary characteristic h(x)complementary characteristic h(x)such that g(h(x)) = Gx

For this to be true G h(x) = g-1(x)g(x)h( )

Gx

For this to be true, G h(x) g (x)h(x)

hx Gxh(x)h gx Gxh(x)

Pre-Correction Features

The FPGA has three correction sections in the forward path:

Envelope equalization: Corrects for filtering effects in the modulator (envelope magnitude and phase response versus frequency)

AM/AM Correction: Corrects for amplitude error inAM/AM Correction: Corrects for amplitude error in the modulator due to capacitive effects in the FET. (Essentially AM distortion)

AM/PM Correction: Corrects for phase error in the pRF amplifier due to capacitive effects in the RF FET. (IQM or IPM effects)

Additionally it will be possible to correct for linear effects in the AM antenna system using a filter in the DSP

Typical Correction Curves AM/AM

Gai

n

Envelope Voltage

Typical Correction Curves AM/PM

rrec

tion

Pha

se C

or

Envelope Voltage

Conclusions

• A fundamental requirement to broadcast the HD Radio signal is to present a symmetrical load to the RFsignal is to present a symmetrical load to the RF amplifiers with Hermitian symmetry.

• A further improvement to the system linearity can be p y ymade by providing correction data curves for the transmitter amplitude and phase responses.

• Technology improvements are making measurement• Technology improvements are making measurement much simpler on the transmitter end, reducing test equipment requirements and making optimization easier.

• Improvements in pre-correction can also help.